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This is a superset, so two exercises are done back-to-back with minimal rest. The first exercise is a major lift done with a partial range of motion and the second is an isolation exercise. How you do your partial reps is important. Keep the target muscle under tension long enough, so perform the partial reps in a "no-acceleration" style. Try to maintain the same fairly slow speed during the whole range of motion, focusing on keeping the tension on the target muscle. It should take you about the same time to complete one partial rep as it would a full rep. Do 8-10 reps on both the partials and the isolation exercise. – Christian Thibaudeau

This is based on the same principle as drop sets: once you hit failure on an exercise, you find a way to continue doing work. With drop sets you reduce the weight so you can get more reps in. With mechanical drop sets you keep the SAME weight on the bar or machine but you change the exercise slightly so that you gain a mechanical advantage and can continue while working the same muscles.

A mechanical drop set can include two or three variations. The order, whether you're using two or three variations, is always to start with the weakest variation and work toward the strongest. That way, when you hit failure on the first movement, you can still get some reps on the next one. Select a load that you can get 6-8 reps with on the first exercise. Take each of the two or three variations used in a set to technical failure or close to it. Remember, you're using the same weight for all the exercises selected. Take 10 seconds of rest when switching between variations. This won't negatively affect any of the growth-producing stimulus but will allow you to get 1-2 more reps per variation. – Christian Thibaudeau

Every rear delt movement I see has the lifter start with his hands and arms in line with their body, then proceeding to move them outward (think of the rear deltoid flyes I showed above). There's nothing wrong with this, other than the fact that solely focusing on this kind of path severely limits how much of your muscle you're going to hit.

To change things up, try setting up on a cable machine for constant tension.
This is a very isolated pattern that, when done correctly, absolutely torches the rear deltoids. As you can see in my execution, it's important to maintain the same elbow angle throughout the entire range of motion. This isn't a triceps exercise; the fulcrum is the shoulder.

Pull the arm all the way across the body as though you're trying to cover your mouth with your biceps. Then aim down toward your outside shoe with your fist. It'll take a few reps to initially feel, but once you're rolling, you'll get an insane pump in the target region. – Lee Boyce