It's apparently legal in USAPL which is pretty strict, or at least I watched a few guys who used the technique openly and got away with it. I think the key is to lower all the way into your chest and stop completely then have the bar go only up at the press command. I've seen some people not control the bar and dip it down into their chest after the press command which is a no no, or have their chest come up but the bar not move, so it looks like it dips which is also a no no. The bar has to be under control at all times and only move up after the press.
A guy in my flight was really good at it and he explained to me why he does it. He said he lowers an inch or so into his chest, then at the press command he drives and flexs his chest muscles which is supposed to break the inertia of the bar by pushing it out of his chest, thereby getting a nudge on the bar to help get it started. Seemed to work well for him. He pressed 335@198
I am Anton Zdravko Martin!
Best meet lifts: Sq 150 Kg (330 lb), Bench 120 Kg (264), DL 160 (352) @89 Kg (197)
Best gym lifts: Sq 375, Bench 280 (pause), DL 385 @205 or less
Goals beyond my wildest dreams, 600 Kg raw total, 200/160/240
Hack away at anything which isn't essential. Do what you love, and do it often. Fazc.