This is an exercise I'm using for an athlete rehabbing a L GH joint dislocation.
Pretty simple, it's I guess what you would call a modified scorpion.
Lying prone, shoulders & elbows @ 90-90.
The athlete reaches posterior left with their RLE & returns. This creates a chain reaction through the shoulder that challenges GH stability safely in what we used to call "closed chain". You may increase/decrease difficulty by raising/lowering the arm/elbow angle. You may also choose to do some self mobilization to the GH joint by placing a rolled up face towel beneath the proximal humerus. This provides a gentle posterior glide to the humerus in the glenoid as the leg comes around.