Execution:

The air squat begins with the feet slightly wider than hip width apart. The distance between the feet should be judged based on mobility (through the hip and ankle), flexibility and comfort. The feet shouldn’t be too wide as achieving correct depth will be hindered, nor too narrow. There is the option to slightly face the toes outward to help with range of motion.
The chest should stay forward facing and tall. If you had a word written across your shirt it should be readable by someone opposite you during the whole movement.
Initiation of the squat requires the weight of the body to be centred over the heels and through the glutes and hamstrings, not the toes and quads. Your hips move back as if you are sitting toward a chair.
As you lower yourself you need to push your knees out and keep your back tall. The bottom of the squat is achieved when the crease of your hips is below your knees.
In the bottom position you should feel the weight distributed towards your heels and not your toes. On the ascent, drive through your heels, push your knees out and finish with the hips, knees and shoulders in one line.

Purpose:

The air squat is the foundation of all the squatting movements. It helps develop strength, control and is an essential fundamental movement. It develops the glutes, hamstrings, quads and core and it can help to improve long term health.

Variations:

The air squat can be helped and aided by use of props like squatting towards a wall ball or bench while developing the flexibility, mobility and strength needed to perform the full movement. The air squat leads into other squatting movements such as the front squat, back squat, overhead squat and goblet squat.