The battery powered beacon goes on the start/finish and the display stays with the bike. It automatically records lap times and can store up to 64 laps to be reviewed later. As per the description, it is accurate to within 0.02s/lap.Hmm... so how do these work? You place the beacon on the start line and it records when you pass by? I wonder how accurate it is vs a GPS based timer (Ie. phone)