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Nothing is weighing wheels or anything like that.
There are two systems, TPMS is the pressure (and temperature) monitor, which is an actual sensor in each wheel. The values are displayed directly.
There is an additional system that compares the wheel rotation, using the ABS sensors. These are monitored continuously and the pulses are statistically analyzed to determine whether a wheel is slightly or grossly different from the others, or what it was measured at previously. This is a more severe kind of trigger.
The owner's manual describes them, the first is called "TPM", Tire Pressure Monitor, and the second is called "FTM", Flat Tire Monitor. The same symbol lights up on the dash for both if a bad reading is detected.
Both systems reset in a similar fashion, only while driving. This is mostly because the TPMS sensors don't send signals unless they're rotating. The FTM monitoring, of course, also needs to be moving. In both cases, the road needs to be reasonably straight and smooth, but only somewhat. The statistical averaging takes care of the details.
Recent Volvos do everything with the second system, btw. And they are quite uncanny about it. They don't read out the actual pressure, obviously, but they quickly detect a low tire, and identify it on a diagram.
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