When modern football helmets were introduced, they all but eliminated traumatic skull fractures caused by blunt force impacts. Mounting evidence, however, suggests that concussions are caused by a different type of head motion, namely brain and skull rotation. Stanford engineers have produced a collection of results that suggest that current helmet-testing equipment and techniques are not optimized for evaluating these additional injury-causing elements.
The brain is like a bowl of Jell-O. Give the bowl a push, and the dessert takes some time to react, but once it gets moving, the Jell-O overshoots its limits and begins wiggling back and forth. This period of brain movement within the skull is a potentially dangerous time for injury to occur. Acceleration is important, but so is the timing of deceleration. If the bowl moves forward right as the Jell-O is decelerating backward, it could cause the Jell-O to deform even further.
The same is true for the brain moving within the skull. It's possible that injury happens when the head whips back and accelerates the brain in one direction right as the brain is starting to go in the opposite direction. A slingshot-like test system is being used that propels an impactor into a stationary head to test impact velocities and accelerations in six degrees of freedom.