Execution
While performing an exercise such as a chest press. Select a weight that will challenge you to perform 8-12 reps with good form. When the muscles have reached the point of fatigue, strip some weight off and complete a few more reps to allow the muscles to work with a lighter weight for a few more reps. You can strip the weight continually until you have reached the point of just using the bar. However I recommend stripping the weight no more than once or twice each set to allow the muscles to use a weight heavy enough to challenge the muscle to become stronger rather than for muscular endurance. I generally recommend doing drop sets with machines as it allows you to strip the weight faster allowing for less rest time and more of a challenge placed on the muscles involved in the particular exercise
Example
While performing a biceps curl, the person lifting the weight would start with a 25 pound dumbbell and do as many repetitions as possible without significantly compromising form. Then a 20 pound weight would be used until exhaustion is reached. As I have already mentioned, you can keep dropping the weight down to the 1 lb dumbbells but I usually recommend dropping the weight to the 20lb and possibly the 15lb dumbbells. This will allow you to focus on muscular hypertrophy.
Variations
There are many variations to this principle. One way to mix up the intensity of the exercise to do a prescribed amount of reps at each weight even if muscular failure hasn't been reached. This is commonly referred to as "down the rack" and is very effective for exercises such as dumbbell biceps curls, dumbbell flys etc. The weight which each succeeding drop set is stripped is another way to adjust the intensity and vary the exercise. A "wide drop set" is one with a larger percentage of weight stripped (usually 30% or more) while a "tight drop set" the weight reduction would typically be from 10-20%.
Cautions
As it is very easy to "over train" with drop sets, it is highly recommended that no more than one to two drop sets be done per muscle group on any given workout. This technique is also not recommended as a long term regimen. The primary focus on drop setting is to "shock" the muscles by adding stress, thus increasing additional hypertrophy A Rose By Any Other Name... Drop sets and the technique also go by the names breakdowns, descending sets, triple-drops (when a total of three different weights are used), down the rack or running the rack (when using dumbbells), up the stack (because with a weight machine, the pin is moved up the stack of plates with each drop in weight), strip sets (when you "strip" weights off the ends of a bar), or the stripping technique (so called because of "stripping" weight plates off with each drop in weight. No matter how you slice it, dissect it or call it, this advanced training principle will help you break thru any plateau, challenge you to reach new progress and leave you feeling like you had a good workout the next day!
*Always refer to your physician and a CERTIFIED Personal Trainer for correct recommendations and proper instruction.