Maybe I described it incorrectly, or got confused xDD
He can control the weight of his own fist(and by extent speed). Maybe increasing the gravity at the end of the punch does nothing, but increasing the speed first will. Haha didn't really attempt to keep it realistic in my original post, as it is anime-based.
Also, he wouldn't be throwing boxing punches or hay-makers with this technique, as he uses Wing-Chun, essentially all his punches will be One-Inch Punches, so he wouldn't really throw himself off balance. :P