The Haynes manual says something like - pop the top off and use a flat head screwdriver to check that the piston (which is threaded) moves back and forth smoothly. Replace the sealing washer when finished checking. The task is marked as 'four spanners' which means it is challenging.
It seems to me like it relies on a spring to automagically take care of things.
In reality, if it failed or got stuck, it could be curtains for the engine. devilsyam's adjuster would need to be checked periodically, but removes the possibility of the engine automagically lunching itself.