A mechanical drop set is a long compound set that combines two to four versions of one exercise. Begin with the simplest exercise and work your way up to the most difficult. When you reach a point where you can't finish any more solid reps on one exercise, you can switch to a new version where you're stronger and can continue to do reps with the same weight.
This allows you to maintain loading the target muscle even if it is too exhausted to continue with the first workout. More muscle fibers will be fatigued as a result of this. Changing angles also attracts various fibers. As a result, muscle mass has increased.
On the first exercise variant, start with 8-12 reps. On the remaining motions, do as many good reps as you can. For triceps, here's an example:
Triceps to forehead extension (use a controlled eccentric or negative)
Pullover and extension of the triceps
45 seconds of rest after the close-grip press