The default setting is on. The two side prep PIR sensors provide wider sensing angle and detect more potential triggers. In some situations, you only want to monitor a narrow spot. Too many irrelevant triggers by the side sensors outside of that spot will keep the camera on and off. This wastes the battery power unnecessarily. In some other situations you may have difficulty removing the interfering tree branches, or avoiding the sunlight. If so, you have the option to turn off the side sensors.
