Its how they press the direction dpad so quickly. Instead of just pressing with your finger directly down, they "roll" bottom of the controller while having your finger pressing just hard enough on the direction you want it to go, and every "roll" is an input movement. They are not allowed to use modified controllers so this is the fastest way to move your tetris pieces during the high speed portions.

Here is a interesting video on the topic: https://www.youtube.com/watch?v=n-BZ5-Q48lE

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.