I've seen it suggested, that the pressure from the wand, can adequately aggitate the carpet fibres and release the soiling.
My personal experience, has been, prespraying with m/s, followed by extraction with u/m in the tank, gives a good result.
I look forward to trying out Nemesis and am considering buying a product from the US which we used in the past.
You should really experiment and find out what suites your particular approach. No two of us, will tackle the job in exactly the same way.
robbie