Vipreet (Viparita) Raja Yoga is the "reverse" Raja Yoga: it forms when the lord of a difficult house (6th, 8th or 12th) is placed in another difficult house. Two negatives cancel, giving a strong rise - often after struggle or through crises that the native turns to advantage.
There are three named forms: Harsha (6th lord in 6th, 8th or 12th), Sarala (8th lord in 6th, 8th or 12th) and Vimala (12th lord in 6th, 8th or 12th). Each turns a dusthana lordship into a source of eventual success, courage or purification. Early life may feel harder; later chapters often show the rise.
Vipreet is the opposite geometry of a Kendra–Trikona Raja Yoga: dusthana lords feeding other dusthanas, not kendra lords joining trikona lords. A planet in the 6th, 8th or 12th is not automatically Vipreet - that is only Trika placement until the lord of one difficult house sits in another. Harsha, Sarala and Vimala name which dusthana lord is doing the work. The rise is typically late, in that lord's own Dasha, after a harder apprenticeship. If the dusthana lord is destroyed, there is no reverse gift left to harvest. Presence of Vipreet does not cancel a genuine raja yoga elsewhere; charts are allowed to carry both.