S&S Cycle Manual Compression Release 90-4925 Designed to be used in S&amp;S cylinder heads for Twin Cam, 4in. bore Evolution big twin, and Super Sidewinder Plus style S&amp;S engines Consists of a small valve that opens during starting and allows a portion of the cylinder pressure to escape into the exhaust port of the cylinder head Reduced cylinder pressure makes it easier for the starter to turn the engine When the engine starts, the valve closes and the engine runs normally, with full compression Manual compression releases are activated by a cable similar to a choke control