To understand the illustrations, it's best to first understand how lift works.
On the left part of the first illustration:
On a 45-degree angle, in a flat body position, less wind will travel across the top of the body. More wind will support the bottom part of the body. Essentially, this will pull your body down quickly.
On the right part of the first illustration:
On the 45-degree, with an arched body position, a flyer has the ability to fight gravity by creating more forward drive. The trick is to try getting as much wind as possible to travel across the top of the body to encourage the lift property. When the momentum picks up, it has more of an affect by dialing in the correct angle and arch.
See the AirPressure.gif illustration: The position is comparible to that of an aircraft wing, if you look at it from the side. Planes lift off of the ground, more because of the wind above the wing than the wind below it.
Sorry Bet Midler, maybe you should rewrite the song.
The arching can be seen through the majority of this video by Marco in Italy: