The Select Button

The select button is most famous for "mashing to keep hitting with Elec Beam" against Yellow Devil, but it does more than that.

The intended function of the Select button is to pause the game without entering the menu.

The specific trick against Yellow Devil is that enemy Invincibility Frames "run out" during a select pause. This allows the player to "mash" select (though it's better pressed with timing) to hit most bosses multiple times.

The select button can also be used to stop knockback with quick presses in places, and is in fact intentionally done in speedruns.