Letís assume that ball travels from opponentís racquet to your point of contact Ttr seconds and your back and forward swing takes Tsw (can be about 0.5sec). Thus you should start backward swing after Ttr-Tsw when the ball lives the opponent racket. Thatís why there is practically no correlation between timing of your swing and ballís bounce.
To illustrate this idea there is Safin difficult FH from baseline http://youtu.be/PvkLy7vZyy4. He almost finishes his forward swing before ballís bounce.