Drop sets have an amazing history. This method was created from the original HIT (High Intensity Training) Methodology and its creator Arthur Jones. Jones created Nautilus Equipment which was technically considered a machine-based company. HIT became a format that coincided with the pieces of equipment he created.
What Nautilus found was that when you isolate a muscle group there is really only one way to maximize performance; by training to complete muscular failure. This is limited with free weight resistance such as with Barbells, Dumbbells, and Kettlebells. With these implements, we can only go to the level of technique, specifically concentric or overcoming gravity, we have. Machines allow us to reach complete failure by reaching concentric failure, then isometric failure, then eccentric failure. With free weight exercises such as squat, hinge, push and pull we have to be more conservative with how we reach our maximal threshold because we can get hurt if we don't.
Jones opened the door for methods that completely exhaust a muscle group. What is so interesting about Drop Sets, is that it allows us to push past concentric failure at a certain exercise by lowering the weight intra-set. Without a machine, we have to find creative ways to completely exhaust a movement pattern that is ultimately limited by our concentric limits.
The Mechanical Drop Set method is built on choosing an exercise for a body part and starting with higher RPE (Rate of percieved effort) and lower rep scheme. Then adjusting the lever we use for a second exercise and lowering the RPE and increasing the rep scheme. Finally well drop the RPE and change the lever one final time to perform our highest rep scheme.
What a Set looks like is:
6 reps, RPE = 8-9
12 reps, RPE = 6-8
24 reps, RPE = 4-6
This works out to be 40 total reps. However, making the lever easier and lowering RPE will allows us, ideally, to maintain good technique longer and get more value from a set.
Certain muscle fibers are more susceptible to fatigue than others. Fast-twitch fibers will fatigue faster than slow-twitch fibers. Using heavier intensities creates more fatigue with fast-twitch fibers more rapidly. If we diminish our reserves of energy for fast-twitch fibers we diminish our value from that set. Drop Sets allow us to override this.
With Drop Sets (or any protocol) it comes down to getting the most from each exercise by orchestrating the right loaded position combined with a certain intensity (load) and volume (sets & reps).
If we continue to do this strategically and prioritize quality, over time we get more recurring value.