Drop sets keep the muscle working for a long period.

Drop sets entail basically doing 3-4 working sets all at once. While you will be lowering the weight each time, you will still be working the muscle. It will not be getting a break from being under tension, and this is what will help you.

Not only will your muscles get used to working without REST, but by being under constant tension it will rip and tear the muscle more easily to help them grow bigger.

Related Questions:

1. What is the optimal rest time to work on endurance?