When the engine starts and the camshaft is rotating at speed the weight/s will sling out. This change in position of the weight/s moves the decompressor rocker arm or pin into a position where it will not contact exhaust valve or rocker arm and the engine will have full compression.educing the engine compression reduces the force required to kick start or pull start the engine, and allows the use of smaller less powerful electric starting components. Decompressor systems require maintenance and inspection to ensure correct function.