[Tervuren]

As force is increased, so is friction.

Think of sliding a 200lb object across ice, now try sliding a 400lb object across the ice. Even if you have the object on a low friction surface, the 400lb object will have more drag on the ground.

You increase power, either by increasing speed(RPM), or Torque. The faster the transmission moves, the faster the frictional parts come apart and together, increasing friction.

There are reasons several forms of US racing use systems that do not change gears, more power to the wheels this way with less moving parts.