Although the MSH-6 MS capsule appears to be just one microphone, it actually contains two mic elements, positioned directly on top of one another. One of the microphones inside the MSH-6 is unidirectional (that is, it is mostly sensitive to signal coming from directly in front, and less sensitive to signal coming from behind or from the sides) while the other is bi-directional (that is, it is mostly sensitive to signals coming from directly in front and directly behind, and less sensitive to signal coming from the sides).
The basic concept behind MS recording is that the Mid microphone picks up signal coming from the center, while the Side microphone creates ambience and directionality by adding or subtracting information from either side.
The Mid-Side technique works well whenever you need a variable amount of room sound. It can also be used for studio recording, and its convenience and flexibility make it a good option for rehearsal and live concert recording as well.