The sensor is motion activated, which means that as long as the sensor is charged, it will start recording data as soon as you horse starts moving, and automatically stop recording once the horse has not been in motion for a couple of minutes. You also do not need to be riding with your phone in your pocket as all of the data is stored into the device. However, to use the GPS function, you will need to manually start and stop your session and keep your phone with you while riding. Simply open up the app and press 'start session'.