Ed, the difference in the Limb Driver is that the rest is spring-loaded "up", much like a regular prong-type rest; string tension pivots the rest downward towards the shelf (look at the far-right in Dan's picture). When you draw and the limbs are pulled inward, the LimbDriver string goes slack and the helical spring pops the rest up. It stays up throughout 80% of the shot, and only comes down when the upper limb is nearly back at it's original position and re-applies tension to the spring, pivoting the rest back down.

Hope that helps explain it better for you.