A decompression assembly (12) and method can include a linkage mechanism (20), a biasing assembly (38) for normally biasing the linkage mechanism toward a locked position when an internal combustion engine (10) is in an unpowered state, and an actuating piston (44) for driving the linkage mechanism from the locked position toward the unlocked position when the engine (10) is in a powered state. The engine can include a cam lobe (70) for driving pivotal oscillation of a rocker arm (36) engageable with a valve (16) in the engine for movement between open and closed positions. The linkage mechanism allows a full range of movement of the rocker arm when in an unlocked position, and restricts movement of the rocker arm to less than a full range of movement when the linkage mechanism is in a locked position for maintaining the valve in a partially open position.