When in Live View mode it will continuously draw flashing outlines on the subjects that are in focus. So all you have to do is manually focus until the desired subject is painted with these outlines. This was the first time I used it but had read about it being added to the NEX cameras and thought it sounded interesting. I was amazed at just how effective it is. No problem at all quickly dialing in focus while shallow DOF.
Thanks! I was wondering about this feature as well and was struggling to find more details from the magic lantern website.
1DX, 24mm f1.4L II, 35mm f1.4L, 50mm f1.2L, 85mm f1.2L II, 135mm f2L, 24-70mm f2.8L II, 70-200mm f2.8L IS II : D800, D4, and a whole bunch of Nikon lenses