Moke, yes. It's a three-microphone recording technique which can be used either for surround recording or for two-channel stereo. When used for two-channel stereo it resembles M/S in the usual sense, except that it takes away one of the big limitations of two-microphone M/S recording.
With two-microphone M/S, you can make one main adjustment in playback: You can vary the proportion of the S signal relative to the M signal going into the matrix. Increasing S gives you more stereo separation and at the same time, increases the proportion of reverberant sound in the result. Decreasing S gives you more and more of a mono signal until finally there is no more S, and the result really is mono. At that point you also have a maximum of direct sound pickup as compared to reverberant sound.
That's very good--generally speaking, a clearer, "drier" sound is preferable for mono--but sometimes you might like to adjust the stereo image width and also the amount of direct vs. reverberant sound energy with separate, independent control over both aspects of the recording. That's what the third microphone and the additional matrixing give you. It's like being able to adjust both the angle between a coincident pair of microphones and their polar pattern--both "after the fact," with the raw recording already stored safely on your hard drive.
--best regards