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 was going to call this article "6 Things I Hate," then I realized I'd be a hypocrite. I tell my seven-year-old daughter all the time that hate is a strong word and it should be used with caution. Hence the new title.