I signed up for a gym and with it I received a few personal training sessions. One of the exercises I was showed was this:
Instead of doing a typical dumbell bench press while laying on a bench...you have your upper back supported on a medicine ball and force yourself to stay straight...as if you were laying on a bench. Once you're in this position, you would perform a dumbell bench press like usual.
My trainer claimed this burned a lot more calories because of the fact that you're using more of your muscles to stabilize yourself and keep yourself up. I'm a bit skeptical that this burned a significant amount of calories more that would justfiy making the exercise so much harder.