The lifting force of all kites is produced by deflecting the air downward, the resulting change in momentum producing an upward force. The reason for this is that the air traveling over the top of the curved surface of the kite is going faster than the air passing underneath. Fast-moving air creates less pressure; this means there is more pressure underneath the kite, and this helps to force it upwards.
Think of the kite as a sail boat, catching the air. The air tries to push the kite along like the sail boat. But the string the flyer holds keeps the kite tethered. Since the kite cannot go with the wind flow and the kite is tilted so that the air is deflected downward, the kite has no where to go but up.