A mechanical drop set is where you combine a few variations of an exercise into one long set. Start with the exercise variation you're weakest at and work your way up to the strongest. That way, when you hit a point where you can't complete more good reps on one exercise, you move on to different variation where you're stronger so you can continue to do reps with the same weight.
This allows you to continue to load the target muscle even when that muscle is too tired to keep performing the first exercise. This will fatigue more muscle fibers. Also, changing angles recruits different fibers.
Start with 8-12 reps on the first variation and do as many solid reps as possible on the other exercises.