Ground Cancel Jump
A ground cancel jump is a jump that does not update Jak's last known grounded position.
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 update Jak's last known grounded position. By jumping during this 3 frame window, Jak will go airborne again and his position will not be updated. These jumps can be chained to move Jak without updating his last known grounded position. By doing this, the game thinks that Jak is continuously airborne.
The ground cancel jump was discovered by Kuitar.