I'm def. not reffering to the legal and well practiced arch of a competitive PLer. More the raising the hips and butt up from the bench in a fuk-thrusting motion as the bar reaches the chest. Appart from seriously affecting the satbilty of the disks in the lower back, this drains all of the stress from the chest and shorteds the ROM considerably. Fair enough if you're doing a new 5RM and your but comes up an inch on the last rep, but bucking and fliping around like a fish just to lift a heavier weight is ridiculous and dangerous.
A few years ago I saw these 3 Sweedish guys (new at my University gym) doing B.P. and their hips were explosively leaving the bench by about 12-14 inches. They were only benching about 245 for a single, but were confidentlty looking over at me with 225 (5x5) like they were the new hot shit. After my 5x5, without a spotter, I casually loaded up 315 and did one of their butt thrusting reps - it was easy because I only had to lower the bar about 5 inches!