Arms in a Y to light up the lower traps and shoulders.
Keep the arms at 45 degrees forming the Y, lift from the shoulder blades not the neck.
The Prone Y's is a Back-first movement, with Shoulders working as the supporting cast. Track it consistently and you'll see exactly how it feeds your back volume for the week.
The Prone Y's primarily works the Back with Shoulders assisting.
Shorten the range or use an easier variation, and stop 1 to 2 reps before failure. Reps build fast when you train just shy of your limit.
For most lifters 3 to 4 working sets. Pick the rep range that fits your goal and progress the load when the top of the range feels owned.
