You may not know the National Operating Committee on Standards for Athletic Equipment by its name or even by its acronym -- NOCSAE -- but if you wear a helmet to play baseball at the high school or college level you know their stamp of approval.
It's the small permanent sticker in the shape of a square that is required on all headgear worn by athletes who play in the NCAA or in the National Federation of State High School Associations. Those organizations do not create their own standards for the equipment, only mandate that all of it adhere to the NOCSAE's guidelines.
To meet those standards, baseball and softball helmets are impact tested by the NOCSAE from six directions at various temperatures. Baseballs are shot at the helmet from a distance of 2 feet at a speed of 55 miles per hour while instruments inside the helmet record the severity index (or SI), a scale used to determine the concussion tolerance. Any helmet with an SI of 1200 or higher will not meet the standards, although that is mostly to protect against skull fractures and bleeding on the brain. It still leaves plenty of room for concussions, which can occur at an SI of 300.
Likewise, although helmet shifting is anticipated in the testing, any structural changes or other changes that result in unrestorable loosening of the fit are cause for failure.
If a helmet passes the NOCSAE's standards, it gets stamped with the helmet-shaped approval sticker, another warning sticker, and the model is ready to be sold and used on a college or high school baseball diamond.