Reed Switch
WARNING: This page is about a different car, the 2006 Dodge Viper. However, it is still accessible from the selected car via links, so may be relevant.
A magnet is used to close the reed switch. The only time that the reed switch is used is when the TPM module is reprogrammed. When a magnet is placed over the end of the valve stem for six seconds, it will close the reed switch. With the reed switch closed the transmitter will send its ID and tire pressure to the module. The ID and tire pressure can be read using the DRBIII®.