Electrically operated. Large displacement / high compression Twin Cam motors with S&S cylinder heads can sometimes be troublesome to start. These electrical operated compression release valves are opened by two separate solenoids; when the starterbutton is activated. When starter button is released the release valves close. Note 1: Will only work with S&S Twin Cam style heads. OEM heads will not have enough material in critical areas to allow machining. Note 2: All 89cc S&S TC cylinder heads are machined at the S&S factory. Machining on the 79cc S&S TC heads is optional. Note 3: Will only work with S&S style rocker covers ''with the central hole'' which provides the necessary clearance for the solenoids. Note 4: For replacement parts see 531408 (compression release assembly); 531407 (wiring harness) and 531422 (silicone solenoid caps).