The standard band external rotation exercise has a few drawbacks. It's easy to compensate by using the lumbar spine. Not good. It's also hard to tell if you're getting full range of motion.
These problems can be solved by doing this supine variation. It's impossible to compensate with the lower back and you'll know when you're reaching full ROM when your wrist bones touch the floor.
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?