Below you can see a diagram of the valve, valve spring, rotocap and camshaft of the OM61X engines.
Below the spring you can see the rotocap or valve rotater. Below you can see a picture of a rotocap:
It is spring-loaded and it acts like a turntable. Every time the valve is pushed open and the valve spring is compressed, the rotocap will turn a little bit causing the valve spring and valve spring cap to turn too. Because the valve spring cap is locked together with the valve itself by means of a notch, the valve will turn too. This is done to prevent an uneven wear on the valve and valve seat. If the valve wasn't turned, it would move up and down in one position, causing an uneven wear.
The problem is that the rotocap itself can wear out, so that the valve isn't turned anymore. That means that the valve and valve seat can wear out unevenly and the valve only is really closed in one position. If you adjust the valves in this situation without the use of the third wrench to hold the valve spring cap, it is possible to turn the valve, away from its position where it closes completely. That means that the valve clearance may change and, more importantly, there is lack of compression.
Here you can see the valve spring cap with the notch which locks it to the valve itself:
here you can see an old rotocap being removed:
Here you can see a new rotocap installed at the base: