The engine is then turned over with a starting handle placed in the starting dog on the flywheel. This is aided by the decompression valve, which decompresses the engine and makes it easier to turn over to allow the flywheel to gain speed and momentum to turn the engine through compression, and get the engine to fire. A spiral groove on the perimeter of the flywheel carries a wheel on the decompressor mechanism and is used to determine the number of revolutions before the decompressor disengages. This is generally up to three revolutions, but can be anything up to six revolutions. Depending on the condition of individual tractors, it may need considerable physical exertion to start a Marshall.