Air swimming

A Grunt air swims. It can be seen in the video around the 01:41 mark. -XDkTflQu5k

Air Swimming is a glitch glitch when a Jackal (or anything that's killable, but Jackals get it more often) is killed by a grenade explosion and their body gets stuck between two objects like Covenant boxes.

The glitch happens where the body is too big to fit in between and so the body hovers in the air between the two objects and looks like it's trying to swim.

Note
In Halo: Combat Evolved, when bodies are flown up by means of grenades, they will appear sometimes frozen if they hit against an object in flight and will appear to be swimming or floating in the air.