I have been doing it for quite a while and you probably have too, that is in order to set the rounds to the back wall of the mag. Since my G19/gen4 is fairly new with tough magazine springs, when I have 15 rounds in there it seems that smacking it a bit against my opposite hand isn't doing much. Perhaps it really is insignificant anyway, and maybe its just a silly habit! Anyway, what do you think? Is it possible to slam off a round by wacking it too hard?