The C-Stick is the yellow analog stick on the right side of the GameCube controller and on the top of the right side of the New Nintendo 3DS. In Super Smash Bros. Melee's versus mode and in Super Smash Bros. for Nintendo 3DS, it can be used to perform any smash attack while standing, or any aerial attack while in midair. With the option to customize controls in Super Smash Bros. Brawl and Super Smash Bros. for Wii U (Smash 3DS doesn't allow to customize the C-stick), the C-stick can also be assigned many other control behaviors, such as inputting tilt attacks (A-sticking) or special moves (B-sticking). The right Control Stick on the Wii U GamePad and Classic Controllers also works like the C-stick for the Smash games these controllers are compatible with.
Smash attacks inputted with the C-stick are performed one frame slower than the fastest possible, though this has little to no effect on its usefulness. If one presses a grab button right after the player tilts the stick, they can charge the smash attack. It is also possible to charge a smash attack by pressing an attack button when tilting the C-stick, in the same manner the player would if they were using the Control Stick, or in Super Smash Bros. 4, by simply holding the C-stick in the direction it was tilted. The C-stick can also be used to perform aerial attacks, in which case the attack will be performed in the direction the C-stick is tilted, allowing the player to do an aerial while moving in another direction without losing momentum, such as a back aerial or up aerial while moving forward. In all games, the C-stick can substitute the Control stick in some instances, as it can be used to roll or sidestep out of shield, determine the direction of a tech, and aid in directional influence (both DI and SDI). The C-stick is additionally notable as the means to perform a DACUS and other advanced techniques.
In Melee's single player modes, the C-stick can be used to change the zoom distance of the camera, by tilting it up to zoom in and down to zoom out. This is essentially pointless, as the camera control cannot do much other than restrict the player's view, and players fond of using the C-Stick to attack will find themselves instead taking no action.
In Brawl and Smash U, functions that can be assigned to the C-stick are:
In Melee and Brawl, the player can also use the C-stick to change the camera angle on the various menus within the game.
These are some techniques that can be performed with the C-Stick that cannot be performed using the Control Stick and attack button.
Aerial attacks in opposite directions
Using the C-Stick, the player is able to move through the air in one direction, but input their aerial attack in another direction. For example, a player could move right, but their attack could be inputted so its hitboxes are on the left side.
Smash directional influence can be performed by using both the Control Stick and the C-Stick. By using both joysticks together, the player can SDI at a much farther distance than possible with just one stick.
Aerials without moving
Using the Control Stick to perform a forward aerial or back aerial will result in the player moving slightly in the direction inputted. However, the C-Stick does not require a direction on the Control Stick to attack with an aerial, so when used with the C-Stick, the player will not move horizontally. Good for countering approaches.
Mashing out of grabs (Brawl)
In Super Smash Bros. Brawl, the C-stick counts as two inputs in the grab escape formula, letting the player escape quicker from grabs by mashing with the C-stick.