For a kite to fly it needs to be pushed away from you by the wind while you pull it back. This harnessing of the winds power can be achieved only if you are upwind of the kite. If you stand downwind of the kite at any point there is no longer any resistance for the kite to pull against and it will stall (flop about in the sky). This does not, however, apply to five-line kites which, due to a design feature, can sit slightly upwind and remain in the sky.
The area in which the kite can fly is actually larger than only directly downwind of you. It fans out at nearly 150° horizontally in front of you, and 900 from the ground to above your head. At the edges of this window the kite will produce the least amount of power. it follows that, flying in the middle of this window, the kite will generate the most amount of power. The 40 percent in the middle of the window is called the ‘power zone’.