In this example, let's assume a 4fm point. 1/5/6 are MMP's, and are therefore in person D. 2/3/4/7 are FMP's in a diamond: 2 guards the breakside, 3 and 4 are underneath, and 7 is deep.
As the disc is swung to 2, our nearest defender marks and stays with this person. 3/4 slide, but stay in the diamond, as does 7.
Let's say they then got a swing off to 3. The nearest defender would slide to mark them and stay in person, while the other underneath defender would maintain the bracket. From here, if any FMP receives the disc that isn't already marked (in this case 4 and 7), we would then be out of our diamond and into person.