Each of these buttons has a tiny curved disk attached to its bottom. This disk is very conductive. When the button is pressed, the disk is pushed against a thin conductive strip mounted on the controller's circuit board.

If the button is pressed lightly, the bottom part of the curved disk is all that touches the strip, increasing the level of conductivity slightly. As the button is pressed harder, more of the disk comes into contact with the strip, gradually increasing the level of conductivity.

This varying degree of conductivity makes the buttons pressure-sensitive (meaning pushing the button harder does have some merit to it)!