Shake Smash is a function in Super Smash Bros. Brawl and Super Smash Bros. for Wii U that allows the player to shake the Wii remote to perform smash attacks as though it were a C-stick. Deactivated by default, it requires custom controls to be activated, with only the Wii remote and Nunchuk combo control scheme supporting shake smash.
The Shake Smash is essentially using the Wiimote as a C-stick, combining the normal attack button plus the direction the remote is swung. Therefore, a sideways shake will result in a forward smash, jerking the Wiimote upwards performs an up smash, and jerking the Wiimote downwards results in a down smash.
As Shake Smash is analogous to the C-stick, it can be used for many functions outside of smashes, such as for performing throws and throwing items. In particular, Shake Smash can be used in lieu of button mashing for escaping from throws, and it can potentially be more efficient than pure button mashing on other controllers.
The Wii remote's motion-sensitive functionality, while downplayed in Brawl, is actually one of the main selling points of the console. Numerous other games on the Wii featured motion controls of some sort, as did some games on the Wii U that could support the Wii remote or the Wii U Gamepad's limited motion controls.
The inclusion of motion controls in Super Smash Bros. Brawl was initially debated among the game's designers. Late Nintendo president Satoru Iwata wanted Brawl to feature no integration of the Wii remote's motion capabilities, feeling that it would interfere with gameplay. Despite this, the designers of Brawl still tried to find some limited uses for it. Initially, shaking the Wii Remote was supposed to cause players to taunt, but it was later decided to use the motion controls to instead perform smash attacks.