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 load as you fatigue, the load stays the same throughout. 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. Here are eight of the best, along with examples for each.
1 Isolation → Compound
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. As an example, start by doing lying 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.
Other examples of this technique include starting with dumbbell flyes and finishing up with dumbbell presses for chest, or starting with chest supported bent-arm reverse flyes followed by chest-supported rows for the upper back.
2 Change Your Grip
On certain upper body exercises, even a slight change in grip and/or hand position can really alter its difficulty, so switching grips as you fatigue can be a great way to extend the set. For example, you could start with pronated (overhand) pull-ups and then switch to neutral grip chin-ups before finishing up with supinated (underhand) chin-ups. This will not only allow you to crank out more reps than you'd otherwise be able to get, but because each grip biases certain muscles differently, it allows you to reap the benefits of all three at once. If you have rings, you can transition from one grip to the next without ever letting go of the rings. I call these "3 way" ring chins.
You can also use the rings for a push-up mechanical drop set where you start with modified planche push-ups and segue into regular ring pushups.
Other examples of this method include starting with close-grip bench presses and finishing with regular-width bench presses and close-grip barbell curls followed by wide-grip curls. You can either go on to failure with each grip before moving on to an easier grip, or you can do a predetermined number of reps with each grip, stopping short of failure on the first part (or parts) of the set. I prefer the latter method, but both can work.
3 Literal "Drop" Sets
For bodyweight exercises like push-ups, inverted rows, and ring flyes, you can start by doing reps with your feet elevated on a bench and then drop your feet to the floor when you can no longer do any reps. For pushups and ring flyes, you can even take it one step further and do reps from your knees. In fact, this three-phase ring fly drop set is right up there as one of the hardest things I've ever tried for chest.
For inverted rows, you can start with your feet elevated and then switch to doing them with your feet on the floor with the legs straight. You can take them one step further by then bending the legs and continuing.
4 Shorten the Lever Arm
Certain exercises allow you to shorten the lever arm to make them easier. For example, bent-arm lateral raises are significantly easier than doing lateral raises with straight arms, so you can start with straight arms and then bend them slightly as you fatigue to keep the set going. Similarly, you could start by doing dumbbell flyes with nearly straight arms and then move to bent-arm flyes with your arms bent to approximately 90 degrees. To take it one step further, you could even go straight into dumbbell presses to really have your pecs give you the finger. Here's a similar mechanical drop set using ring flyes instead of dumbbell flyes, going from full flyes to bent-arm flyes to pushups.
For abs, you could do hanging leg raises with straight legs right into bent-leg raises. Same idea.
5 Tight Form → Looser Form
I'm not an advocate of sloppy and reckless form, but it's at times acceptable on certain exercises to use a little bit of momentum to help keep the set going as you start to fizzle out. With shoulders you can start with strict overhead presses before using some leg drive towards the end of the set to knock out some push presses. This doesn't give you free reign to start leaning way back and cranking out hideously ugly (and dangerous) cheat reps, it just means using your legs a little bit to get the bar moving.
For biceps, you can start with strict curls before moving to power curls where you use the hips to create a little bit of momentum to help move the weight. Again, this doesn't give you license to start leaning way back and heaving the weight up. Another option for shoulders is starting with strict lateral raises and then moving to "cheat" laterals as you near the end of the set. Just don't go overboard. If the people around you start to wonder if you're having a seizure, you've gone too far.
6 Unilateral → Bilateral
Here you'd start with a single leg exercise and then transition directly into a bilateral exercise.
Since you're usually stronger on the bilateral exercise, a good rule of thumb is to do twice as many reps on the bilateral exercise as you do on the single leg exercise. You can of course adjust the reps according to how strong you are on each exercise, but I've given you a good place to start. For example, do five single leg hip thrusts on each side followed immediately by 10 bilateral hip thrusts.
Similarly, you can start with single-leg Romanian deadlifts (RDLs) and transition to bilateral RDLs immediately after. A lot of people find that balance is the limiting factor with single leg RDLs, so this allows you to nail the technique down when you're fresh and then finish off your hamstrings with bilateral RDLs to ensure that you still get a good training effect. Another good one is doing reverse lunges and moving right into squats or front squats, depending on how you load the lunges.
7 Alter Stance Width
Start with narrow-stance paused squats and then continue with wider stance squats without a pause at the bottom (though still making sure to use controlled reps). This can work well with both front squats and back squats.
For trap bar deadlifts, you can start with "duck stance" deadlifts (heels together and your toes pointed out at an angle) before switching to regular-stance trap bar deadlifts.
8 Change Bar Position
Start with front squats and then rack the bar briefly. Put the bar on your back and keep squatting. This is incredibly taxing, so save it for the end of a lower body workout because you won't feel like doing much of anything afterwards.
While mechanical drop sets can be a good way to add muscle to stubborn body parts, it's important not to go overboard with them. A little moderation goes a long way. Two to four sets should be adequate for upper body exercises, while 1-2 sets is plenty for lower body. Also, don't go crazy and start adding in mechanical drop sets for every muscle group. Limit yourself to 1-2 per workout.