Over on AT somebody was saying that the holding weight only increased by 1 to 2 lbs.

Working the math backwards
70lb bow w/ 80% cam = 14 lbs holding weight
14+2lbs of holding weight = 16lbs / .2 = 80lb draw weight

They also mentioned that on one of the tested bows the difference in the limb tip position was about 1/4" shorter on each side when using the device. That extra 1/4" of deflection comes at the maximum loading of the limbs so more speed still.

Seems pretty easy to see where all of the extra speed comes from although to get the energy out, you still need to put it in. There is no free lunch.