How do they work?

Dashcams record video to a removable memory card inside the camera itself. Any time your car is on and the dashcam is plugged in, it is acting as your backup witness. Dashcams automatically start recording when they are powered on, and stop recording once you turn your car off. Note: Some dashcam models have the ability to keep recording even while the vehicle is parked! This is known as "parking mode".