Video motion detection (VMD) is the most commonly used method for event-based recording in a surveillance system. Mainly because it’s so easy to setup. By creating motion windows within the camera’s viewing area and embedding defined parameters, specific events can be captured based on what moves within the motion windows.
Accurate motion detection can cut your storage by more than 80%, and bandwidth by the same amount if recording is being sent to an external device.
A word of caution...two things you need to be aware of.