The Midair Shell Jump, commonly referred to as simply a Midair, and also known as the TAS Jump is widely considered to be one of the most difficult to pull off tricks in Super Mario Maker.
How It Works
A Midair Shell Jump is done by throwing the shell backwards and redirecting the shell forwards before Mario's throwing animation finishes. When Mario starts his throwing animation there is 1 frame before the shell is thrown. By turning around during this frame the shell is flicked underneath Mario.