How the motion sensor works:
V2/Classic SkyBell: The motion sensor will activate when someone stands in front of SkyBell for over 10 seconds. This will ring the bell and send a notification to your phone even if they do not press the button.
HD: Within the app under "settings" you can select the sensitivity of the motion sensor, high will trigger a call quickly (within 10 seconds), medium (approx. 20 seconds) and low will trigger a call with about 30 seconds of motion.