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.

Nothing. That's the whole point of the topic. It's all nonsense.

Stuff like that doesn't happen in real racing. In real racing you usually pass someone by pressuring them to make a mistake or forcing them to make an opening in a corner. Some motor sports have boost mechanics like DRS or electric boost and stuff but those are limited to specific areas with specific rules on their use. It's not the winning strategy usually.