Koroyd is a rotational impact protection system made from welded polymer tubes that crumple on impact to absorb energy. Unlike MIPS, which uses a sliding layer between your head and the helmet liner, Koroyd is built directly into the helmet's foam structure itself, replacing traditional EPS foam in key impact zones.
The big advantage of Koroyd is that it absorbs both linear and rotational forces while allowing excellent ventilation — those hollow tubes let air flow through the helmet far better than solid foam. You'll find it mainly in premium Smith helmets.
If you run hot or ride in warmer spring conditions, Koroyd's breathability is a real benefit. It's proven technology for energy absorption, though it's less widely available than MIPS. Either system is a solid choice for rotational protection — pick based on fit and comfort, and don't skip this type of protection entirely.