Koroyd is a rotational protection technology made from welded polymer tubes that crumple on impact to absorb energy. You'll spot it easily—it looks like a honeycomb of small tubes visible through the helmet vents, giving Smith helmets and a few other brands their distinctive look. Unlike MIPS, which uses a sliding liner, Koroyd physically collapses to absorb both linear and rotational forces.
The big advantage of Koroyd is weight. It's notably lighter than many other protection systems while still offering rotational energy absorption. It also tends to allow excellent airflow through those open tube structures. However, it provides somewhat less rotational protection than premium systems like MIPS Spherical or WaveCel.
Koroyd makes the most sense if you're looking at Smith helmets and want a lightweight, well-ventilated lid with more protection than a basic EPS foam helmet. It's a solid choice for road and gravel riders who prioritize weight and airflow but still want rotational protection beyond standard foam.