Our geocentric vantage-point enables us to witness the phenomenon known as retrogression as planets go around the sun in their cyclical orbits. Planets that normally appear to move from Aries towards Taurus, at times, appear to move backwards, from Pisces towards Aquarius and so on. The Sun and Moon are immune to this apparent backward motion. The mean lunar nodes always move backwards, but the true nodes describe a bi-directional motion somewhat like other planets; though the nodes are mostly retrograde but at times travel forward - what is known as direct motion in astrology.
The reason for this apparent retrograde movement of planets varies. For the inferior planets, Mercury and Venus, when they are between earth and the Sun, they appear to move retrograde; however, when they are on the other side of the sun (Sun is between the planet and earth), they appear direct.
Superior planets, Mars, Jupiter and Saturn are retrograde when they are in signs opposite to the Sun, in other words, when they are just ahead of the earth and we are trying to catch up with them until such time when they are a bit behind the earth in the ever going orbital race. The remaining time they are in direct motion. It is perhaps an interesting astrological symbolic clue that the phenomenon of retrogression is not a real one, but a product of our perspective or vantage!
Many views exist on the astrological implications of such apparently retrograde phases of planetary motion. Most astrologers agree that at least subtly different effects are produced by the same planet when it is in direct in contrast to retrograde motion. Retrograde Mercury is anecdotally associated with communication breakdowns, more computer troubles, etc., and causes much anguish in certain circles! Some astrologers tend to put too much emphasis on the retrogression phenomenon.
In Vedic astrological treatises, retrogression has been considered as an indication of strength (chestabal is the technical term used for the strength when a planet is retrograde). Some experts qualify further by saying that the natural benefics such as Venus, Mercury and Jupiter are strong while natural malefics such as Saturn and Mars are weak or evil when retrograde, unless Saturn or Mars happen to be ascendant lords or yogakarakas (by virtue of being lords of angular and trinal houses). Nothing more elaborate is generally made of the phenomenon of retrogression in most classics.
In practice, retrograde planets generally seem to affect the mental plane more than the physical or worldly plane (perception of what is apparent is often a 'mental' thing!). During the dasha (planetary period) of a retrograde planet, one would be hard pressed to find a difference in the types of events that occur or other physical effects; however, the person with the dasha lord retrograde in his chart may react to the goings-on differently than another individual undergoing the dasha of the same planet in direct motion.
Those with retrograde planets often respond to 'time' in a nonstandard way. This is perhaps symbolically linked with the concept of their being related with "other spheres of time," the brave romantic thinker may stretch that to include the past; even earlier reincarnations that expose the nativity to "karmic" influences. The concept of karma generally is considered to be associated with linear time. The retrograde planets expose one to influences that are not a part of the currently perceived reality, in other words, sensory and other 'inputs' emanating from "other" realities are able to make it through normal barriers and are able to affect "existing perceived reality" and ones decisions. In a sense, retrograde planets open up, or at least perturb some kind of a "temporal reality-boundary". This in itself is neither good nor bad. Contrary to popular opinion, I have not come across an instance when the dasha of a retrograde planet produced a major "transforming" influence that could be unequivocally attributed to "retrogression" itself. Some believe that retrograde planets only manifest strongly in the lives of those who have advanced sufficiently on the spiritual path and have the heightened inner capability and sensitivity to perceive and express the energies that the retrograde planets represent. Others experience the effects in a more subtle manner.
Each planet represents certain attributes and natural portfolios. Likewise, each planet holds sway over certain houses in a horoscope. By viewing retrograde planets against such a background of information, interesting insights can often be achieved.
Taking retrograde Jupiter as an example, a notable thing about nativities with a retrograde Jupiter almost anywhere in the chart but particularly in the 3rd, 4th, 6th, 8th and 11th is that they are presented with opportunities in life to bring to fruition other peoples' failed projects. Also, retrograde Jupiter may sometimes affect the gonads and/or growth-related hormones, particularly the pituitary and thyroid. Obviously, there would not be an obvious disease or defect in all, but, in many the functioning of these glands might lie closer to the extremes of the normal range indicating a borderline or latent disorder that may remain clinically unnoticed and perfectly compatible with normal growth, etc.
From the perspective of karma, expansion seems to be one of the significant 'life-tasks' for nativities with a retrograde Jupiter. The symbolism is obvious! The individual is presented with opportunities for expansion (of body, mind, spirit, ideologies; of boundaries in general). Indeed one is almost 'forced' into situations that would cause expansion. What one does with that 'expansion' is where the arguably present faculty of freewill perhaps comes into play, leading to either a promotion to the next grade in the school of life or an opportunity to take another go at the task perhaps through reincarnation.
Retrograde planets signify a mental state that contains certain highlighted areas or unusually heightened traits. With Jupiter retrograde, there is seen a tendency towards over utilizing of the psychological defense mechanism known as "reaction-formation". This makes one take an exaggerated "opposite" stance to whatever it is that triggers discomfort or anxiety. A retrograde Jupiter can often affect in ones inner life an area that has remained unattended, undeveloped or unexpanded! A diametrically opposite area in such a situation may be seen to be overemphasized as if to compensate for the deficiencies. Obviously, the placement and rulership of Jupiter in such charts would give a clue as to the areas that are so affected. With recognition of such areas, some of the psychic energy can be diverted from the zone of abundance to that of deprivation. In a sense, one might say, that a retrograde Jupiter could reflect some of the traits and vibrations that are representative of the spectrum of the "energy" that we normally associate with the resonance characteristics of the planet Saturn, in it being an equalizer, a re-distributor, a leveler.
We may visualize each planet as having a certain energy that flows easily through certain channels or 'jurisdictions', these 'areas' are recognized in astrology as the natural significations of the planets. Mercury, for instance, enables 'cosmic' energy to flow into pursuits that are intellectual, that depend on reasoning, sequential intellectual processes, demonstrating a connectivity that strikes many of us as a logical A to B to C to D sequence and which most of us generally find easier to follow. Mercury does not very comfortably handle an A to C or A to D jump or even a C to A back flip (a C to B to A would be handled easily by it though). Therefore, the prime issue is not the direction of logic but the sequential steps involved. Rapidly commuting from A to B to C to D in a typical mercurial fashion would appear to be governed by intuition. However, if we were to take away one of the intermediate logical pegs, then Mercury would begin to stumble becoming increasingly unable to process information effectively. When Mercury stumbles, being intrinsically impatient, it might be tempted to fill in the gaps with assumptions or even by confabulation (at its worst, an afflicted Mercury has been known to represent a habitual liar, a liar that actually believes in the lies he or she is telling at the moment).
Some astrologers consider a retrograde planet as a 'blocked' or deficient responder! By that token, one with a retrograde Mercury would not find logical thinking very easy or even something within one's capability. The retrograde planet appears to move backward in the zodiac due to our geocentric viewpoint and it is easy to fall into the "similia similibus" trap and consider the apparent backward motion as something 'negative' because of its symbolic parallelism with the connotation of the term 'backward'! Perhaps it is not very simple and the retrograde planet is merely shifting its modus operandi to an alternate modality. Perhaps during the periods and transits of retrograde planets, one needs to undergo a "paradigm shift", in other words, to embark on an alternate approach, using a very different viewpoint of looking at the same thing. Would a retrograde Mercury, then, be more 'comfortable' in using an A to C or even an A to D approach, rather than the usual A to B to C to D that it is 'expected' to? A 'normally' logical, sequentially thinking scientist with a retrograde Mercury may find such 'intuitive' jumps a more efficient way of overcoming 'blocked progress' when doing an experiment or examining a hypothesis.
There are in allegories (dry facts presented in a more interesting and palatable format!) a hint of the possibility of reasoning and logic (Mercury), and intuition (Moon) together forming a complete information sensing and processing unit. Indeed, the mind that has harnessed and can utilize both sequential reasoning and intuitive jumps, progresses a lot faster, better and deeper than one that has only mastered either mode of gaining information. It must be noted that Moon and Mercury can only bring facts to one through different channels or means, but one still needs the blessings of Jupiter to incorporate such 'information' into wisdom. It is often seen that the individual with a retrograde Mercury is more comfortable or more accepting of modalities of fact-gathering and presentation that do not necessarily rely on the usual sequential approaches.
It is important to remember that the end result or the core-nature of what a planet represents remain the same regardless of its apparent motion in the radix. The direct Mercury would be more comfortable with sequential processing of information; whereas, the retrograde Mercury would tend to explore the dimension of non-sequential intuitive processing. In either case, Mercury would still be gathering and processing information. In other words, regardless of its apparent motion, Mercury remains associated with thinking and information-processing. The difference between the direct and retrograde versions being in the way the mode of processing and thinking (reason and logic vs. intuition) operates.
Retrogression could represent an intrinsic strength in ones being or may represent an armor with cracks in it, the outcome depending upon how well the retrograde planet is connected with other factors in the chart. In the end, polemics and occasional flashes of intuitions aside, the 'weight-of-evidence' approach is what saves the day for most astrologers. Curious as it might be, retrogression, is but a single factor or phenomenon and not the critical card moving which topples the entire structure. After all, neither the horoscope nor the human life experience is a house of cards!