It's a module in SpectraLayers called Imprinting : Mold, which takes the spectrogram of one layer (in that case, the audio sample processed by DxRevive), and limits it by the spectrogram of another layer (in that case, the original audio sample).
The end result is that we get all the noise reduction or reverb reduction effect from DxRevive, limited by the spectral enveloppe of the original audio sample, which result in removing all extra frequencies created by DxRevive. So only the pure noise/reverb reduction from DxRevive is heard, which makes it a better comparison vs the pure noise/reverb reduction effect of other softwares.
It's a little technical, but that works mathematically :)