Mechanical drop sets (unlike regular ones where you just reduce the weight as you fatigue) allow you to switch to an easier modification of the movement, usually with the same weight. This helps you to increase metabolic stress and time under tension, both of which will stimulate hypertrophy.

Try this mechanical drop set out for chest:

1. Grab a set of dumbbells that you could do 12-15 reps with. You'll want to be conservative here. It's best done with hex dumbbells.
2. Knock off 8-10 flyes with good form, staying well short of failure.
3. On your last rep, without stopping, squeeze the two dumbbells together in the top position and complete 8 to 10 reps of the squeeze press. Squeeze hard and maintain constant tension on the pecs.
4. After the last rep, again without stopping, try to get another 8 to 10 reps with regular dumbbell presses. Don't be surprised if you fall a bit short.

If you use this as an intensity technique during the main part of the workout, do 3-4 mechanical drop sets. If you use this as a finisher for your chest workout, then one set is more than enough.