Below is a drawing, taken from the RTA manual, of the reversing servo motor.
The reversing servo motor is only used for the fuel pump cam as the exhaust valve cam shape is identical in both directions and does not need to be moved when reversing the engine.
In the drawing the the fuel pump cam is being driven by the wings identified as item "11" which are attached to the camshaft. The whole assembly is being driven in the direction shown by arrow "DR".
Whilst the cam is in this position space "RA" is kept pressured with oil from the Crosshead LO system through connection "VA".
To change the direction of the engine, oil from the Crosshead LO system is directed through connection "VB" into the space marked "RB". As the cam is is moved the oil contained in space "VA" is released out through connection "VR".
It is important to remember when carrying out a camshaft inspection that there is no pressure in either space "RA" or "RB". Because of this the fuel cam itself is free to move and will do so without warning. This can trap any hands or ther objects which are in the way. This movement of the fuel cam can also cause some worrying bangs to be heard when the engine is turned for crankcase work, and if not prepared can start engineers looking all over for the source.