The tyre pressure monitor works via the wheel speed sensors of the ABS. With decreasing tyre pressure, the rolling radius of the respective wheel is reduced and the wheel rotates faster for the same car speed.
The tyre pressure monitor helps the driver to keep a check on tyre pressures. Continuous monitoring ensures a high level of safety. The correct tyre inflation pressure extends the tyre life and fuel consumption is lower.
Note: All the same, the tyre pressure monitor does not exempt the driver from his responsibility of regularly checking tyre pressures.