Off Loading Compressor
CTF, I know that driving the Compressor takes a lot of Energy which is extracted by the associated turbine, however I do not know of any engine which has other than normal bleed air extracted during reverse thrust operation. The compressor bleed valve remains closed as far as I am aware. Also, the RB211 is limited to 90% N1 RPM at Maximum Reverse Thrust, a high percentage of normal limiting N1 which is around 104-106%.
To me, the term "Reverse Thrust" says it all and I relate it to Vectored Thrust. The engine is fixed in place and the only variable is the direction in which the mass air flow is vectored. I am not convinced by the "Inlet Drag" theory although I accept and appreciate the high degree of engine "power" taken to drive the compressors and ancillary items driven by the accesory gearbox.