If you crank over an engine by hand, you will notice that the force required to turn the crankshaft is not constant. For a timing belt the same is true. As the cylinders and camshafts rotate, the pistons compress air and the lobes open the valves. These changes in force require tension on the timing belt to be constant and even. In this video, Andrew Markel discusses how the engine can kick back during stop/start operation and the role of the tensioner. This video is sponsored by Litens.