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