As one of the Airline Engineering guys who applied a ‘calculated tap’ to the Spey I can tell you that it was the CSDS PRV that we ‘tapped’. The mechanism was prone to sticking. I have seen various implements used, including a chock. The start up was always a waiting game to see if the engine started or the CSDS Drive sheared. Prior to this happening you would hear a ‘zing’ as it sheared leaving you in no doubt what the problem was.