Nothing to do with gears. When you're drafting behind an opponent you can accelerate slightly faster than them because you have less wind resistance. You can also reach a higher top speed before the wind resistance is too much for your engine to overcome. Building up enough speed in the draft can let you make an overtake or get alongside before the next corner.

Whether it is actually an effective way to pass depends on the type of car and track. It's an important part of many types of racing but totally ineffective in others.