Drop sets are a time-tested, muscle-building technique. You take a weight and rep it out, reduce the weight and rep out again, and then reduce the weight further and do it again. Essentially, you reduce the weight as a means to keep the set going.
Mechanical drop sets follow a similar concept, only rather than reduce the weight as you fatigue, the weight stays the same. You simply switch to an easier exercise or an easier variation of the same exercise to extend the set further.
The key is to use exercises that sequence well together and require little-to-no setup changes so you can transition from one exercise to the next with minimal disruption to the flow of the set. There are many different and effective ways to employ mechanical drop sets for both upper body and lower body work. Try this one for triceps.
Triceps: Mechanical Drop Set
Here we'll use lying "dead stop" triceps extensions, then move to close-grip presses. Start by doing the triceps extensions with a weight that lets you get 8-12 reps. Then transition immediately into close-grip presses for the same number of reps that you got on the extensions.
This version of the mechanical drop set method involves going from an isolation exercise to a compound exercise. Start with an isolation exercise and once you can't get any more reps, move directly into a compound exercise that hits the same muscle.
Another example of this technique is starting with dumbbell flyes and finishing up with dumbbell presses for chest.