In a crash, one of the things that damages the brain is straightforward impact; another is violent twisting of the brain within the skull, because of the raggedy ways that riders typically land. Rotational protection are features that try to let the helmet absorb some of that violent twisting, so that your head doesn't have to. MIPS is one of the most common technologies - a thin plastic lattice that the helmets cushioning pads sits on, which can twist a few millimeters within the helmet. MIPS Spherical allows more substantial movements between bigger layers of the helmet. Some brands - such as POC - have their own technologies. And other technologies, such as the honeycomb Koroyd, combine crumple protection with a degree of twist protection.