synaptics touchpad

I was wondering how the Synaptics Touchpad on laptops receive input. Is it heat or pressure? I want to use the touchpad like a drawing tablet, and I was wondering if it was possible.

It uses electricity. It's on Wikipedia so it must be true.
http://en.wikipedia.org/wiki/TouchPad
TL;DR: you can't.

Threadjack: someone know if it's reasonably easy to rip out a touchpad from an old laptop and use it as a standard pointing device in another machine?