When should black make the cxd4 move in French? [FEN ""]
1. e4 e6 2. d4 d5 


Following the structures with black pawn on c5, there is a tension in the center. When does black usually decide to take the d4 pawn, what is usually the trigger?
Generally speaking, resolving tension helps the defender. White is the defender as far as the d4 square is concerned so Black should not aim to play c5xd4 unless he gains some follow-up advantage by doing so. One such advantage might be access to the b4 square for a knight or bishop after the reply c3xd4.