With mechanical drop sets, instead of dropping down to lighter weights when you reach failure, you keep the same weights but switch to a related exercise where you have a mechanical advantage. This allows you extend the set, triggering metabolic stress and increasing time under tension, both crucial for hypertrophy.
Here's an example for arms:
Start with 8-10 Zottman curls. Curl the weight up normally and then rotate your wrist into a palms-down position to lower the weight.
Now do 8-10 hammer curls without resting.
Finally, rep out with 8-10 alternating dumbbell curls.
You multitaskers out there will love this movement as it allows you to work the rectus abdominis, the obliques, and the serratus at the same time. Too bad the rest of your life isn't as efficient.