The 'pumping' could just mean (and I think it is) that when there is a note on that frequency it quickly sucks all the power from the power unit capacitors (while still not producing any reasonable sound pressure in the end), and there is much less power left for producing other sounds just few moments after this note is started. Something like that in simple words.
I need to test it more but I've been so happy with it disabled that I haven't been inclined to follow through.
As an example, if someone on the track started a sentence with the word 'but', it would sound like - bUT. The 'bah' part would start at out one volume level, then the 'UT' would suddenly jump up to a very obvious degree. If the sound track in general was busy, then of course it would be less noticeable, but as soon as a quiet portion is interupted with something, it would be very obvious to the ear. A tiny lag, but still impossible to ignore.