Headwhiffing is a technique in Super Smash Bros for Wii U and Super Smash Bros Ultimate. Formerly known as Header Momentum Exploit (HME), headwhiffing can be described as a method of Header canceling in the air without going into free fall.
Just like Header canceling, headwhiffing places the ball right next to Wii Fit Trainer, ready to be hit by a different attack. To perform the tech, Wii Fit Trainer must first be launched by a strong attack. Once this has happened, the player must input Header immediately out of hitstun. If the knockback Wii Fit Trainer has received is sufficient, she will miss the ball with her headbutt due to her momentum. From here, the player can follow the falling ball and hit it with an aerial or even Super Hoop. The most notable use of this tech is headwhiff dair, which sends the ball almost straight down and as such can be used to cover Wii Fit Trainer's landing.
The downsides to headwhiffing are its notable start-up time and its expenditure of Wii Fit Trainer's only Header use until landing. However, the start-up time becomes a nonissue if the tech is used extremely far off or above the stage, as it usually is.