Think of this exercise as a push-up that's mostly for your shoulders instead of your chest. It forces the shoulders to work hard to drive your body back using a natural arc and push angle.
The Push-Back Push-Up
Level 1: Drop down, explode straight back, and return to push-up position.
Level 2: Same basic movement, but put your feet up on a braced bench. Your abs will be screaming.
Back in my small hometown in Texas, some of the more eccentric churches would hold these great tent revivals. Even if we didn't like their particular style of religion, we would still sit out in our cars and watch just for the sheer entertainment value of the show.