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.
Arnold Schwarzenegger did a lot of things wrong. He performed endless sets of leg extensions thinking that he was "cutting up" his thighs. He believed that pullovers would spread the rib cage and pull-ups would widen the shoulder girdle by spreading the scapulae.