Each site is a little different:

At Prima it works on how much you put into the pot.. So lets say you are getting 40% rakeback.

Theres 10 players.. The pot is 40$ you put in 20$ of that pot (So 50% of the pot) and the rake is 2$.. Since you put in 50% of the pot, you get 'credited' for 50% of the rake or 1$ of that 1$, 40 cents would come back to you in rakeback.

Hope that makes sense