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.
Chances are if you're over 30 that backside of yours is starting to look like two gluten-free pancakes, even if you train. Here's why and how to fix it.