Here's a question for the electronics experts: how do the tire pressure sensors work?

We have an '09 that has this feature. My wife drives the car almost exclusively, and yesterday the tire pressure light came on. She went into semi-panic mode about it, so today I checked the tire pressures. (So I need an idiot light on the dashboard to remind me like I need a nagging wife, right?) Typically I over inflate the tires, but I hadn't checked them in a while. I found the lowest at 28 lbs. (hardly under-inflated) and the highest at about 34 lbs. I inflated them all to about 40 lbs. and the light went out.

So what is this system monitoring? Actual tire pressure? Or divergence between pressures in the tires based upon standing height?