Retrograde is the apparent motion of a graha that is going backwards in its travels around the sun as viewed from the earth. So if you went out tonight and found Jupiter, it is 16° Scorpio. Then looked at Jupiter say the 5th of August, Jupiter would be 15°58' Scorpio, as if it was going backwards in the zodiac.
This has been the condition of Jupiter for some months now, and it just so happens it begins its forward motion this month, 6th of August at 9:35 GMT if my calculations are correct. I mention this time as Jupiter is then neither going back or forward, some say sama, even-ness or stand still and a perfect time to experience turiya for those that practice meditative techniques. As Jupiter is the owner of Akasha,and is Brihaspiti, Guru is in a Moksha house of Scorpio, it is a very favorable time , samdhya occurs..
Any way back to retro. We say it is retrograde because if my vantage point was a top view say above the Sun and I was able to view all the graha's movement, all would be traveling with normal motion around the sun. Some planets more circular then others, and some at an angle to the plane of the sun, but none would be going the opposite way of the pack.
This Retrograde in jyotish gives chesta bala or the strength/power to strive and work hard. [ hard for me is consistency, with focus, alacrity, etc. and the more Sani is involved the work could mean toil and labor]; the operative word here is strive.
This condition may be a mackle [ a blur, or double impression] when considered. or experienced. Sometimes the hard work bares fruit and sometimes it does not. Like all things in Jyotish multiple things have to be weighed accordingly. What are some of these things? Is the graha in retrograde a natural benefic ( like Jupiter) or a malefic like Saturn? And as usual does the graha fall in a trine ( 1st, 5th or 9th), a kendra or angle as its called ( 1,4,7, 10th house) or a dusthana , that of the 6th, 9th, and 12th houses (bhavas).
A retrograde planet in general is an indication of the denial of its fruits ( but not always and every time - I will outline this below). That is , the retrograde graha tends to reverse its indications of its deliverable, expectation and/or its karaka ~ signification.
Lets say one has a retrograde graha in the 11th house, the house of gains. This may indicate that efforts by legitimate means, will not bare fruit. This person may do hard work (chesta bala) consistently, but ends up with meager results. What may occur? going to illegitimate means (a.k.a. Tony Soprano life style perhaps?)
It is considered fortunate if a malefic and retrograde, is a kendra or trine, and debilitated (we call debilitated neecha) this is a blessing one brings forward from a past life. It takes the malefic and reverses its maleficence, to benefic status. Yet , here's the pickle, if this malefic ends up in a dusthana ( 6th , 8th, or 12th) then the opposite is true and you are back to grief that has been brought forward from the past.
Now what is the past? for some, it could mean from childhood, for others it could mean a past birth. To this one looks to divisional charts , the varga charts in Jyotish that do not appear in Western Astrology methods.
So the conditions do matter. Neecha planets are welcomed for the sadhu, and abhorred for the acquisition of material gain. It all depends on ones orientation.... this neecha condition gives some of the most favorarble yogas one can experience. Jaimini says tasmin ucche neecha ca Srimantaha, or if a planet is exalted or debilitated, it will give good and reasonable finance.
Retrograde in Western Astrology
I am no expert on this matter, but its my understanding a retrograde planet then gives aspects to the houses as if the graha was in the last house it occupied. What does this mean? Lets consider the the 7th house, all graha's less ketu as he has no head to 'see' gives a look, an aspect, or dristi, to the 7th house from its current position. So , as Jupiter is in Scorpio as we stated above, it would give an aspect, a look at 100% of its strength to the 7th house from that location , or it would aspect Taurus. So if its retrograde, the Western approach says it aspects as if it was in Libra, the previous house it occupied, and then would give dristi to Aries, the 7th house from Libra, and the 6th house from Scorpio. To this, my parampara does not subscribe to this rule. Perhaps others on HDF have an opinion in this matter.