The goal of a standard drop set is to allow you to continue performing an exercise even when the point of muscle failure (inability to do one more rep) has been reached. When you reach failure, you reduce the weight by 25 to 50% and continue to perform reps. This can be effective, but it isn't optimal.
"Mechanical" drop sets are similar since you're still focusing on doing more reps once you hit failure. However, this time you don't reduce the weight. Rather, you make a small change to the execution of the movement that allows you to get more reps with the same weight. The change can be a difference in grip, foot stance, angle of movement, etc. It's pretty much the same basic exercise or movement pattern, but with a technical variation.
Here's a cool biceps workout that'll introduce you to the fine torture that is mechanical drop setting.
Mechanical Drop Set for Biceps
A1. Steep-Angle Preacher Curl 6-8 reps, then 10 seconds rest as you change the angle of the bench.
Execution: Perform with your elbows on the steep side of the preacher bench using either dumbbells or a straight bar. As soon as you hit failure, turn around to the angled side of the bench.
A2. 45-Degree Preacher Curl Using the same weight, do as many reps as you can. 10 seconds rest.
Execution: Perform the curl on the angled side of the preacher bench using either dumbbells or a straight bar. Move directly to exercise A3 after you reach failure.
A3. Standing Barbell Curl Same weight for as many reps as you can.
Get the idea? See the related links below for more details and workout ideas.