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.
For this anti-extension core exercise, set up in a slight forward lean. Fall forward with no back sag. Shift your weight onto your heels to return to the start.