To be more precise, while sliding your PDA, have a look at the "Xpos" label at the bottom of the screen: when this value reaches 0.0, you have to invert the direction of slide!

SOURCE

http://www.area51staff.it/mobility/XRay.html

Reviews:

** [Sep 23, 2008] by Ritchie Hicks using Touch Diamond
Pointless application. Don't waste your time downloading it.