When you see us talking about rotational impact protection, we're referring to the damage that can be caused when a helmet rotates on your head in the moment of impact. There's a difference between the damage caused by your head hitting a surface, and that caused by your helmet sliding across your head at the same time.
To tackle this and make helmets even more safe, plenty of helmet manufacturers have developed spherical helmet technologies. Imagine a ball and socket, where one layer moves freely, while the other stays put. Research suggests that incorporating this type of protection can absorb more energy and reduce the amount of force on your head.
You'll see lots of helmet brands offering their own takes on spherical technology. Key examples are MIPS, used by a variety of brands, SPIN by POC, and Koroyd by Smith.