How dangerous is arching your back while doing a bench press? I've seen tons of guys in the gym who greatly arch their back off the bench, assumedly to assist with the lift. Is this bad, or just an alternate form?
Here's a link to Ripptone explaining how much to arch the back: http://www.youtube.com/watch?feature=player_embedded&v=lBNeeeTId1M
Some people have a 'big' arch, this is the person using the legs/hips to drive the bar, usually intended to push heavy weight. So, neither is wrong, it depends on what your goal is: building strength or building ego by moving heavy weight. (butt and shoulders should be touching)