Before mark v engine i.e. before 1991 man engines had throttle valves which works to dampen the exhaust valve closing also the thermal expansion of oil is taken care of this valve.
Throttle valve is mounted on actuator high pressure area.
This throttle valve can control amount of oil going in to the hydraulic cylinder of exhaust valve. So, if fully open this will reduce the pressure and also no damping of exhaust valve when closing.
So, if fully open exhaust valve will knock while closing.
If we start closing at one time knocking will stop, turn more than 200 to close valve and adjusting is done. So knocking and thermal expansion taken care of. It is like tappet adjustments in 4-stroke engine but virtually we are adjusting as there is no tappet so it is also called virtual tappet.
After Mark v engines throttle valve is removed.
Damping done by a damping piston in hydraulic cylinder of exhaust valve. Orifice vent plug is provided to take care of thermal expansion. No adjustments done or needed in this arrangement.