I was curious as well and found this video explanation[1]. The idea is that tapping on the back of the controller pushes it up into the player’s thumb which is resting on the D-pad causing a button press.
So a detail I didn't see from the others to add on. Tetris technically allows you to push the button every frame, so 60 moves/second would be optimal if you ignore physical limits of the controller etc. With the oldest way of playing called DAS (I forget what it stands for) after a couple frames it starts inputting really fast, but you have to store inputs to manage to make it move the way you want efficiently. This new rolling is the fastest way found so far to press the direction buttons quickly and get as many presses per second possible to better position blocks.
Delayed Auto Shift. In NES tetris the rate is 10 moves per second, so to beat it in speed you have to press on the dpad at more than 10Hz.
I think you mean 30 moves per second, as Tetris (like most games) doesn't count two consecutive frames of "button pressed" as two button presses, but a press-and-hold. Still faster than the 10Hz of DAS, but by necessity a full, separate button press requires at least one frame of "button pressed" separated by one frame of "button not pressed".
You're right on 30fps. Been a while since I followed NES Tetris. Thanks for the correction.
They hold one finger over the desired button and the finger roll on the back pushes the controller into the first finger, generating multiple taps faster than you could with a single finger.
It's basically like bump firing but for controllers instead of guns. World-class revolver shooters also use this kind of method.
You rest your finger from the other hand on the button in front, so when rolling the button is pressed. That way you can use more than one finger to press, which allows faster pressing. The fastest rollers can hit the button around 20 times a second, whereas by holding the button down (i.e. manipulating DAS, the traditional technique) you can only reach 10 Hz.