If somebody has lower back issues, then one would want their feet elevated to take the pressure off their lower back. You do lose the ability to do more weight but what you get out of the exercise is more core work. Elevating your feet isn't wrong, it is just a modified form of doing regular bench press that you would do if you had say a bulging disc or something wrong with the lower back. As far as the 90 degree rule that someone questioned earlier. If you do exactly 90 degrees, that leaves the bar only about three inches off your chest. It also takes a lot more strength to lower it to that point and then force it up than it would if you were to drop it off your chest and get that bounce. If you hit your chest you're probably doing two things. The first is that you're engaging your rotator cuff when you don't need to. The second is you're probably cheating because your using the natural momentum of the bar bouncing to 'help' you. This is why it's considered cheating. I consider it dangerous to hit your chest because of the rotator issue. I blew out my rotator pretty bad when I was 20 so I had to learn these things the hard way. You may not have to worry about something like that but I do. So, I bring it to 90 degrees or about 3 inches off my chest to avoid further complications
