The thing below is known as a restrictor gate.
It's found inside an arcade joystick to limit the movement of the stick to certain angles (ie. Vertical only, Horizontal only, 4 way, 8 way) and also to stop the full motion of the stick so it doesn't damage the button circuitry inside. (Didn't get a pic cause WZ was hovering around me when I opened up the stick) The gate above is square shaped and pretty much allow movements to all eight main directions (up, down, left, right and the diagonals) It's a very common gate that you find in most sticks. The gate below is a bit uncommon.
It's an octagon shaped gate. While you can also move in all 8 directions with this gate, the main difference is that the stick will pretty much drop into each direction so the player can be more aware that he has pushed into a direction properly.
I wonder if a simple piece of plastic like this can help me do hadokens better when I'm on the left?