We saw several models of mountain bike helmet above that include MIPS. We mentioned it in our reviews, but just to clarify MIPS stands for Multi-Directional Impact Protection System.
It is an additional layer, made of light plastic, which sits inside the helmet and directly against your scalp. This layer provides a few millimeters of movement at the very moment of an impact. This, it is claimed, can help to reduce the chances of a rotational brain injury.
As we mentioned, not all manufacturers are on board with the effectiveness of MIPS, which is why you won’t find it on every model of helmet.