Concussions are odd. They are mainly related to acceleration/deceleration forces, change of head direction, etc. People with longer/weaker necks are at particular risk for concussion, because that leads to rapid head movements - this is why it's such a bad idea for little kids to play tackle football and why females have higher concussion rates than males. So, adding weight to the head, as with a Guardian Cap, may actually increase concussion risks. The research is currently mixed - biomechanical research shows no real advantage of the Caps
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5634228/ but the NFL suggests that concussion rates are down since implementing the Caps (do we believe the NFL? They have a lot of reason$ to claim the Cap works and their sport is safe).