Ретроградные планеты

Retrograde motion indicates the destructive and obstructive forces of karma. Therefore, if any planets are retrograde in the chart, we may expect that a strong desire is connected to that planet. The planet's power increases when it is retrograde or stationary (state of planet just before and after retrogression), but the results get delayed, become disturbed. After ascertaining what role does the given planet play in the chart, we may understand that the native has some unresolved problems or unfulfilled duties in that area, coming from previous life. (с) Gauranga Das, SVJC

Retrograde planets can even deny the fruits of the actions until they become direct in transit. (с) Sanjay Rath

Retrogression is considered as a indication of motional strength i.e., Chesta-Bala.
References from classics: the retrograde planets become strong and confer good results during their periods. When the planet is in retrogression, they acquire more and more motional strength. A retrograde planet attains youth and becomes a king at the fag end of its retrogression. If a retrograde planet occupies any Bhava other than its exaltation, then the native is like a king. He gains much fame and good results.