At the basic level all fitness trackers rely on an accelerometer, measuring acceleration, direction and intensity. These trackers use light to measure pulse rate. They shine a light on your blood vessel and detect the changes in blood volume as your heart beats. Sensors detect the amount of light reflected back. Less reflected light means a higher blood volume. During movement the readings become less accurate. For more prescise measurements I recommend a chest strap. Because the chest strap is over your heart and closer than a reading from your wrist they are more precise.