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Puts a locking pin through a rocker arm to change what a valve does without changing the camshaft. Two systems use the trick: variable valve lift, where the pin selects between a mild and an aggressive cam lobe, and cylinder deactivation, where it unlocks instead so the valve never opens and the cylinder stops breathing. The pin is moved by engine oil pressure admitted by a solenoid — there is no separate fluid — which is why oil level, grade and condition are diagnostic questions on any of its codes rather than maintenance ones. Jammed part-way it can lose contact with its cam lobe and strike it again under valve spring load, which is how these systems damage camshafts and lifters.