Mask Storage is a technique exclusive to Majora's Mask 3D. The principle is based on the fact that transformation from one form to another can be delayed for an indefinite period of time.
If a transformation mask button is pressed while link is in the air, the transformation will be 'stored' until link touches the ground again. The transformation can be delayed even further by holding L and R before landing. This will cause link to bring up his shield as soon as he lands and will not be able to transform until R is released, until link backflips/sidehops or until L is released and repressed.
Normally, when holding L and R the game will force link to put away any item he has out (such as a bottle or a bow). Storing a transformation while in the air will cause link to be able to shield without putting away the item if he holds L and R upon landing.
The concept is extremely useful in multiple situations. This one concept leads to many other glitches, including Down A, Timestop, Any form on epona, Blank B, Ocarina on B, Weird B and more.