Never seen that before looks almost like a tricep extension? I've tried every rear delt exercise my preferences..
1. High rep bent over reverse fly. Mix in "skiers" and rows, just blast them don't worry about sets or reps.
2. Reverse pec deck 20- 30 reps. High rep set, a couple drop sets, then one heavy set.
3. Bent over dumbbell face pulls, I feel this much more than regular cable face pulls.