L-band signals can penetrate through clouds and rain. This property is why L-band is used for GPS and other applications that require all-weather operation, as it allows for accurate data collection even in the presence of adverse weather.
The idea is to help focus brushing on specific teeth or quadrants, and it uses Bluetooth to know when the brush is on or off. Maybe it signals when you push too hard and the brush pauses for a second as well.
To my knowledge it does not know where you are in your mouth when brushing (positioning via accelerometer/gyroscope), so a synchronized start would likely give similar results.