Hi, I just got a new helmet with MIPS and as far as I can tell it’s basically a second, thin plastic layer inside the helmet that’s separate from the foam of the rest of the helmet. This second layer is what contacts your head and is connected to the rest of the helmet by what appear to be a few short, thick rubber bands. These rubber bands allow the outer shell to slide around and rotate a little bit, the idea being that this will lower rotational forces on the head in the event of a crash.
I’m not sure how effective this is, (haven’t crashed yet) especially considering your scalp moves in the same way.