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.
I'm thinking about great, big triceps. Triceps so thick that you'd have to rest them on a desk or chair to keep them from tiring your shoulders and elbows. What did these old-school guys do to get them?