content: When you are watching a police cam, the video starts 30 - 60 seconds before the officer presses the record button.  When the record button is pressed, audio begins to record, but extra video is retained from half a minute preceding the moment of recording.  Officers can only hope to press the button in time to record evidence, so the police-cam is constantly recording and looping over live video in case an officer is a moment too late to record.