Without a probe, they are almost certainly altering the phase of the each channel to remove background noise You can do this with at matched pair of mics and a

Without a probe, they are almost certainly altering the phase of the each channel to remove background noise You can do this with at matched pair of mics and a desk that can flip the phase of channels. https://en.wikipedia.org/wiki/Out_Of_Phase_Stereo https://en.wikipedia.org/wiki/Out_Of_Phase_Stereo