What it means
The 6th, 8th and 12th houses are the difficult ones: debt and enemies, sudden loss, expenses and exile. Viparita means reverse. The idea is that when the lord of one of these houses sits inside another difficult house, the trouble cancels itself out.
Texts name three versions. Harsha: the 6th lord in a difficult house. Sarala: the 8th lord. Vimala: the 12th lord. Each is said to bring a rise after a setback: a win in a dispute, a recovery, a gain through someone else's loss.
This rule accepts any of the three, and it is common: about 6 charts in 10 pass. Which house holds the lord matters more than the pass itself. A caution: the effect is described as arriving through struggle, and usually late. Nobody calls it a comfortable yoga.
If it passes on your chart, look back. Was there a stretch where a loss turned out to be the opening?