Knock Sensor: Description
WARNING: This page is about a different car, the 2004 Kia Amanti. However, it is still accessible from the selected car via links, so may be relevant.
The knock sensor is a piezoelectric device attached to the cylinder block that senses pressure from engine knock conditions. This vibrational pressure is then converted into a voltage signal which is delivered as output. If engine knock occurs, ignition timing is retarded to suppress it.