There are a ton of different training techniques that come and go but the drop set remains one of few that've stood the test of time.
80~ years ago Henry Atkins, editor of Body Culture magazine published an article on the 'multi-poundage system', this is thought to be the origins of the drop set that is every single day in gyms around the world.
The name 'drop set' is reasonably self-explanatory.
You perform your regular set of X reps at X weight and then you immediately drop the weight and continue to rep out the same exercise, generally until failure.
Here are a few examples...
Perform 8 reps of incline dumbbell bench press using 120lb dumbbells before immediately repping out a pair of 60lb dumbbells until failure.
Perform 12 reps per arm of 30lb alternating dumbbell curls before drop setting immediately to a pair of 15lb dumbbells until failure.
A Couple of Common Drop Set Styles
There are dozens of other words guys use to describe specific styles of drop sets relative to the style of weights they're using such as:
Running The Rack
Running the rack involves performing a dumbbell based exercise with a heavy weight for a prescribed number of reps before immediately performing the same exercise with a slighly lighter weight of dumbbell off the rack, continue repping out the weight and dropping it down until you reach the lightest weights on your dumbbell rack and you rep them out until failure.
Here's an example of running the rack...
12 reps of dumbbell shoulder press @ 90lbs
10 reps (failure) of dumbbell shouder press @ 70lbs
7 reps (failure) of dumbbell shouder press @ 55lbs
5 reps (failure) of dumbbell shouder press @ 40lbs
11 reps (failure) of dumbbell shoulder press @ 20lbs
Plate stripping is essentially the barbell variation of the running the rack dumbbell technique we discussed above.
Unlike running the rack which can easily be performed by yourself I recommend having a training partner on hand for your plate stripping sets for time efficiency, this saves you having to get off the bench and run around to each side of the barbell to take weight off.
In order to perform a plate stripping set you'll want to load up your normal weight for the first set of the exercise using smaller plates (as we're going to be stripping off a plate from each set for your drop sets).
Regardless of whether you're benching or curling don't use big plates like 45lbs to make up your usual weight, use a combination of smaller ones - ideally having 4 or 5 plates on each side of your barbell.
Perform your first set for the prescribed number of reps as normal, then have your partner immediately pull 1 plate off each side of your barbell.
Rep it out.
Have your partner pull another plate off of each side.
Rep it out once again...
Repeat until you're repping out an empty barbell 'till failure.
When Should You Perform Drop Sets?
I personally don't recommend performing drop sets every single workout, I recommend saving your drop sets for if and when you have a lagging muscle group.
I personally first implemented drop sets for my barbell biceps curls after lifting for a couple of years,
They just wouldn't grow. I know I have long biceps insertions which means even when well-developed I won't have a monsterous biceps peak.
But regardless of this genetic limitation I couldn't get any growth whatsoever until I implemented plate stripping my biceps curls once a week.
Use drop sets to achieve that extra level of hypertrophy as necessary.
Drop setting every single exercise during every single workout is a sure-fire way to ensure you're not recovered in time for your next workout, greatly increasing your risk of injury.
Can You Use Drop Sets For All Exercises?
Technically there's not an exercise that comes to mind that couldn't be implemented into a drop set.
Everything from weighted pull-ups to rope triceps extension and Romanian deadlifts CAN be performed in a drop set, and I've seen them being done.
But I personally recommend being very careful of the exercise you choose to implement running the rack and plate stripping with.
Think about it.
The more fatigued you become the more your form begins to suffer.
Poor form increases risk of inury.
I personally would not drop set any exercise that places a large amount of stress on your lower back.
As such that rules out the barbell squat and the deadlift.
Run the rack on your chest, shoulder and arm exercises until your gains are content.