In bodybuilding and weight training, using drop sets is a technique for continuing an exercise with a lower weight once muscle failure has been achieved at a higher weight. It is most often performed on weight machines because reducing the weight quickly is thought by some to be extremely important, but it can also be performed with dumbbells and other free weights.
Let's suppose you're doing bicep curls with 125 pounds for a set of 8-12 reps. The 10th rep is difficult. The 11th rep is extremely hard, even with a little cheating. The 12th rep takes an all out supreme effort. Gun to the head - you still couldn't do a 13th rep. You've hit honest failure. But if you strip some weight off the bar - about fifteen to twenty percent or so, you can keep going.
Even though you may reach a point of momentary muscular failure after 8-12 reps in a conventional straight set, you haven't reached absolute failure; you've only reached failure with that poundage. You see, in a single straight set performed to failure, you don't activate every fiber in a muscle group. You only recruit the number of fibers necessary to lift a particular weight for the desired number of repetitions. By stripping off weight and continuing the set, you cumulatively recruit more and more "reserve" muscle fibers.
Drop sets hit the "stubborn" muscle fibers "deep down," causing growth that normally couldn't be achieved by stopping after a single set of six to twelve.