The Gravity Plus Tail Light has a built-in accelerometer that detects deceleration or swerving and automatically changes to a super bright, solid beam to warn following traffic. To allow the accelerometer to calibrate, always install the Gravity+ on your bike before powering it on. Hold the power button down for 1 second to turn the light on or off. Then, keep your bike still for 1 second after pressing the power button to complete the calibration.