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.
This six-week program breaks some of the currently accepted rules for workout volume and duration. That's why you'll do more than 20 sets per workout and you'll train for 60 to 75 minutes.