A mechanical drop set is where you combine 2 to 4 variations of an exercise into one long compound set. Start with the weakest exercise variation 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 of the exercise 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 initial exercise. This will fatigue more muscle fibers. Also, changing angles recruits different fibers. The result? More muscle growth.
Start with 8-12 reps on the first exercise variation. Do as many solid reps as possible on the other movements. Example for triceps: