Koroyd + EPS hybrid liners combine two materials to give you better protection where it matters most. Koroyd is made up of tiny welded tubes that crush on impact, absorbing more energy than standard foam alone. Helmets with this hybrid design place Koroyd in critical impact zones — like the sides and top — while using regular EPS foam in other areas.
The real-world benefit is incremental: you get slightly better impact absorption in those key zones, plus the Koroyd tubes allow more airflow, so the helmet breathes better. You'll notice this most on warm spring days or during high-output touring.
That said, all certified ski helmets meet the same safety standards regardless of liner material. Koroyd is a nice upgrade if you're already looking at premium helmets and want every edge in protection and ventilation, but don't feel pressured into it — a well-fitting EPS helmet with MIPS is still an excellent choice. Just expect to pay more for the Koroyd hybrid, typically found in Smith's higher-end models.