With regular drop sets, you lower the weight as you fatigue to get more reps. With mechanical drop sets, the weight stays the same but you switch to an easier variation of the exercise to get more reps. This is great for creating metabolic stress and increasing time under tension, two keys to muscle growth.
Here's an example for triceps:
Do 10-12 reverse-grip triceps extensions at a high pulley cable station.
Next, turn around and match that rep count with overhead extensions. Hold each rep for a 2-second pause in the stretched position.
Face the stack one more time and do as many regular triceps pressdowns as possible with good form.