I learned of this exercise from Chris Melton over at therotator.com
It is a combination of a face pull with an external rotation plus a y-press. This combination is great for strengthening all the posterior shoulder/scapular muscles.
Here is Chris’ version using resistance tubing:
I tried them using suspension straps (homemade TRX or blast straps) and have to say they are one of the best posterior rotator cuff / upper back exercises I’ve tried in a while.