M/S technology is commonly known as a variant of stereo microphoning. This technique uses a microphone with cardioid pattern for the middle signal (M) and another one with bi-directional pattern with an offset of 90° for the side signal (S). The main advantage of this technology is its mono compatibility. FM radio stations use M/S technology for transmitting stereo signals exactly for this reason.
To create M/S signals, the left and right channel of the stereo sum are added to generate the mid (M), whereas the side (S) is created by subtracting the right from the left channel:
M = L+R
S = L-R
To decode an M/S signal back into stereo again, M is added to S for the left channel and S is subtracted from M for the right channel:
L = M+S
R = M-S
The integration of an M/S encoder and decoder into a saturator generates new potentials that classic stereo processors can hardly offer. One of the main advantages is the possibility to process the middle and side signals separately. This way you can make the center sound fatter without corrupting the original stereo spectrum, for example.
Of course it is also possible to apply heavy distortion to the side signals without affecting the mid at all, and anything in between... The stereo width can be influenced fast and efficiently, too, and it is possible to process specific parts of a mix that could not be 'touched' in a stereo mix as precisely as it is possible in M/S mode.