bench question

[keasbey]

why do i see some guys at the gym bring there legs up and rest them on the bench instead of down? only thing i can think of is it might keep the lower back from coming off the bench.. which brings me to my other question.. is raising your lowerback while benching good or bad form?

 

[fortunatesun]

Some people, usually powerlifters, will use an arched back. This shortens the length the bar has to travel, leaving more strength for the shorter drive back up. Sebastian Burns is a famous example:http://www.ironasylumgym.com/videos/052006sebastianburnsbench.wmv
The rule here is that your hips have to maintain contact with the bench.
Bodybuilders are not as fond of this style as they are looking to maximize muscle recruitment rather than getting the biggest bang for their buck.