Excerpts from the article: The Helmet Wars.

(Foster, T., 2013)

Two helmets in one, an inner shell snug against the head, with an outer shell resembling a traditional helmet, separated by a slippery cushion that allows the outer helmet to rotate independently of the inner helmet, can reduce rotational energy by 55 percent (Foster, T., 2013), making the difference of whether an impact to the head causes concussion. No helmets currently in use can prevent rotation, the primary cause of concussion. Current helmet tests do not attempt to measure rotation, making them useless for predicting their ability to protect against concussion.

For generations, doctors believed that concussions were a sort of bruising of the brain's gray matter at the site of impact and on the opposite side, where the brain presumably bounced off the skull. The reality is not nearly that simple: Concussions happen deep in the brain's white matter when forces transmitted from a big blow strain nerve cells and their connections, the axons. (Foster, T., 2013) 

"Rotational forces strain nerve cells and axons more than linear forces do," Cantu says. "They're not only stretching, but they're twisting at the same time. So they have a potential for causing greater nerve injury." (Foster, T., 2013)

First up in Halldin's test is the non-MIPS helmet. Halldin flips on a high-speed camera and steps back from the impactor, ready to catch the helmet on its rebound. "Five, four, three, two, one" There's a loud clattering as the sled shoots forward at 22 feet per second and the helmet drops to meet it at 12 feet per second -- crack. (Foster, T., 2013)

I can see on the computer that the head sustained about 170 Gs of linear force, and it rotated 14,100 radians per second squared (the standard scientific metric for rotation). It's a big hit, one that would probably result in a concussion or worse.... (Foster, T., 2013)

Now comes the second helmet. Every variable is the same as in the frst test except for the addition of the low-friction MIPS layer. “Five, four, three, two, one…”—crack. This time the computer shows rotation of 6,400 radians per second squared, a 55 percent reduction. (Foster, T., 2013)

He looks at the colorful graphs on the computer screen again. If the test dummy were a football player, he would have just walked away from a game-ending impact without a concussion... (Foster, T., 2013)

Reference:

Foster, T. (2013, January). The helmet wars. Popular Science, 282(1), 50-77.
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