In short, you hold the controller so that you have a finger resting on top of the d-pad, and then with your other hand, you drum your 4 fingers in sequence against the back of the controller, causing the d-pad to press into your finger in extremely rapid succession.
Tangentially to this, I'm part of the speedrun community, where controller legality can be a hot topic on a per-game basis. I think it's equally cool to maximize skill within the restrictions of the original controller as it is to use new hardware to break that barrier. Different communities decide upon what mods or alternate hardware is acceptable based on what they feel is appropriate for getting the most enjoyment out of their game.
In short, you hold the controller so that you have a finger resting on top of the d-pad, and then with your other hand, you drum your 4 fingers in sequence against the back of the controller, causing the d-pad to press into your finger in extremely rapid succession.
Tangentially to this, I'm part of the speedrun community, where controller legality can be a hot topic on a per-game basis. I think it's equally cool to maximize skill within the restrictions of the original controller as it is to use new hardware to break that barrier. Different communities decide upon what mods or alternate hardware is acceptable based on what they feel is appropriate for getting the most enjoyment out of their game.