Casting my mind back to a classroom at the RR Filton factory, I distinctly remember an ex-Shack Air Eng called Len Schofield telling us about the fluidic flip flop valve (Turbine Pressure Ratio Transducer) that compared pressure either side of the LP turbine and ensured that the RHFCU would stop delivering fuel if the ratio got too high or too low. There is even an urban myth that it was intended to form part of the active feedback loop for fuel control in reheat, such that nozzle opens, fuel flow increases, TPRT detects change in pressure ratio, fuel is trimmed accordingly.
It was also a useful tool for fixing a sticky nozzle in the days of vactric motors. Having waggled taxy nozzle a few times, you got the pilot to drop the offending engine into ENC (which often took some explaining), disconnected the RHFCU fuel solenoid, got the slightly concerned pilot to reset the ENC and 'exercise the throttles up to max reheat', explaining that it couldn't light but would 'blow out' because of the TPRT and drop to the normal dry nozzle area. After returning the nozzle to ENC, simply reconnect the RHFCU solenoid, step smartly away to the side of the aircraft and invite the pilot to try again - 8 times out of 10 this would result in heat, flames, noise and a happy pilot
, occasionally followed by a scowl from the SEngO for conducting reheat runs on the pan! Oh the happy days of TWCU.