The timer is started and stopped by the competitor. There is a sensor where each hand is placed, and when the competitor lifts his hands off, it starts the clock. The timer stops when both hands are placed back on the sensors.

Among serious cubers, it is commonly known that the "Rubik's brand" puzzles are some of the crappiest turning puzzles one can get. Anybody who cares at all about speed will go out and get a cube from somewhere else (I buy all of my speedcubes at hknowstore.com).