I think it's called rolling.

Basically, they are holding their fingers and the gamepad surface still and rolling the entire back of the controller around the gamepad surface.

This allows them to move the controllers super fast.

Since NES gamepad buttons and surfaces are pretty small, they are pretty hard to manipulate really quickly. By flipping the controls and moving the back of the controller and keeping the gamepad surface still, they can make bigger movements, faster.

I think you need the insane speed to get near max input speed (I believe max speed that the game enforces is 12 inputs a second but my memory might be wrong.)

Edit: https://youtu.be/n-BZ5-Q48lE that's an explanation of the physical play techniques the best players use to get world records in Nes Tetris.