Divevault

A divevault is when you throw cappy and dive, hit the ground and roll boost just as Mario is vaulting off of cappy. Divevaults help keep horizontal momentum which make them very useful in areas where you want Mario to jump after getting up on a ledge.

Execution

A divevault is performed by diving then pulling back on the control stick just long enough so Mario just hits the ground before hitting the cap but still rolls. The moment Mario hits the ground pressing X or Y is necessary to boost and get speed. When done properly, yellow around Mario will appear and the vault animation will play. The timing for this is pretty tricky and it's hard to describe how to properly do it, but it's just a matter of messing around and practicing until it becomes consistent.