Ocarina dive is a trick which causes link to pull out the ocarina while falling down through the air. The glitch is very useful as it can be used in tricks and techniques such as Captain's Hat early and Song Storage.
There are four main methods to ocarina dive, different ones work on different versions so you should check which version you're playing before you attempt this trick.
Pulling the ocarina without holding a direction or on the first frame possible after shielding will make Link fall without pulling the ocarina.
Alternatively, you could use Zora Link's second slash in his regular combo.
Ocarina dive can be used to open chests surrounded by flames as there is a frame when coming out of the ocarina stance where the chest may be opened. An example of this is Captain's Hat early.
The most common use for Ocarina dive is to perform a technique called Song Storage.