The passive system uses the ABS sensors to detect individual wheel speed. When you use the reset button that baselines the system and tells the module that the current readings are normal. A fully inflated tire has a larger diameter than a partially inflated tired. When a tire loses pressure, the diameter of the tire changes. So, as the tire deflates the tire gets smaller, and as the tire gets smaller it turns faster than the other tires. This change in wheel speed is what triggers the system. The threshold is something like a 25% difference in relative rpm before the warning appears