Manual says pre-collision will: 1. retract the seatbelt if it thinks collision is unavoidable, or if driver is emergency braking or loses control; it's operational if car speed > 3 mph or 5 km/h, or if relative vehicle speed difference with car ahead is > 18 mph or 30 km/h, or if approaching an obstacle at > 18 mph or 30 km/h; 2. apply greater brake force than normal (relative to driver pressure), if it thinks there's a high possibility of collision; it's operational if (a) brake is applied and (b) car speed > 18 mph or 30 km/h, relative vehicle speed difference with another car is > 18 mph or 30 km/h, or car is approaching an obstacle at > 18 mph or 30 km/h. 3. if it thinks there's a high possibility of collision, warn the driver with a warning light, warning display and buzzer, and apply the brakes automatically to reduce vehicle speed. This last can be turned off or on, using the PCS button (looks like it is very low on the dash, just left of the steering wheel column). It can't detect plastic obstacles (e.g. pylons), and may also have trouble with various other items (pedestrians, animals, trees, snowdrifts, motorcycles and bikes).
