Mask Jump is a glitch in Majora's Mask 3D that uses mask storage to interrupt a jumpslash, which gives you some extra height to grab a ledge normally out of reach. Mask Jumps can also be consecutively chained together to give you infinite height or distance. It can be performed with any form that can do a jump attack except for Fierce Deity Link (Child Link, Deku Link with Deku Sword, and Zora Link). Although a Mask Jump can be done with many forms, Zora Link is the only useful form; since Zora Link is the tallest which allows him to grab really high ledges, and the other forms tend to crash after doing multiple Mask Jumps.
The reason this works is because as Zora Link starts his jumpslash animation and takes out his fins you interrupt the animation with mask storage which gives you jumpslash height while keeping you in the freefall state. You need to use the touch screen for the mask button because a touch screen input takes 2 frames to register instead of 1 frame with the regular buttons. The trick works by pressing doing a B input and then using a mask frame perfectly 1 frame after.
Same as home buffer method except press the B button and the mask button with the touch screen at the same time without home buffering
For Child/Deku Link it takes several frames to take out their swords and not 1 like Zora Link, so doing mask jumps is a little bit more difficult. You need to be in the air, press B and then home buffer until you see this frame (frame before link touches his sword):
Once you hit this frame, exit the home menu and hold the mask button with the X or Y button. If successful, Link will make a small hop in the air.
Mask Jump is very useful for Sewers Skip as the setup for it is much more simplified.
Get next to the ledge to perform a Long Jump as Zora (Make sure your fins are put away)
Right before reaching the ledge do one of the aforementioned methods of mask jump above to make a small hop in the air and grab the ledge.
When you perform a mask jump, normally Link can't use any other item or anything until he lands on the ground. But if you perform a mask jump and at the first frame possible open the Mask Inventory and put away the mask you used to do mask jump, you do a Mask Jump and can still use items or even make another jump slash while still in midair.
This comes helpful because this way you can clip into Milk Bar at night to do the Wrong Warp to Stone Tower Temple in the Any% route, skipping having to get Epona.
You can also gain height using mask jumps to reach ledges you can't normally reach.