Air Bubble is an airbending ability that surrounds your body in a bubble of air stopping all incoming projectiles. This ability costs a lot of chi to create and maintain.
It has two actions (indicated in the crosshair), left clicking expands the bubble and destroys it, knocking enemies back and right clicking contracts the bubble, dissipating it.
As with all other abilities, there are two choices for Air Bubble's level IV upgrade.
Pressured bubble: Very large size
With this upgrade, the Air Bubble becomes extremely large in size. It has 14 health points which mean the bubble can resist more projectiles. It's recommended choosing this path for people who use this ability offensively and defensively.
Undercurrent: Limited hovering (disabled through sneaking)
This upgrade allows you to hover over 1 block above your current level Y. It also has 14 health points but is smaller than the previous upgrade. If you want to hover over certain blocks or avoid lava, this path is the one for you.