Ground Cancel Jump
A ground cancel jump, also known as a 3 frame jump, is a jump that keeps Jak in an airborne state.
When Jak lands back on the ground from a jump or from loading a save file, it takes 3 frames of being grounded for the game to register Jak as not ariborne, or as grounded. By jumping during this 3 frame window, Jak will remain in an airborne state. These jumps can be chained to move Jak around in a non-grounded state.
The ground cancel jump was discovered by Kuitar.