Reply: This function realized through the I/O alarm cable, through the equipments in vehicle. There are I/O cable come with MDVR. For example. Set 1CH I/O in cable connect to vehile open door sensor, and set one cameras to be relevanted with this alarm, when the button actived, then will transfer a signal to the DVR and make it alarm, and then monitor will switch to this channel camera and full screen it. When you set camera two for backing vheicle, connect another IO cable with the backing sensor in vehicle, and set cameras 2 to be relevanted with this alarm.when backing car sensor is actived, then monitor will switch to this channel camera and full screen it.