661 EVO Elbow Guard uses D30 material to absorb and disperse the energy created by an impact. The molecules in the D30 material are soft and flexible but lock together if you crash. The greater the force of the impact, the more the molecules lock together and the greater the protection.
These are well made and provide great protection and coverage. How ever I have 12.5 biceps and have mediums and they never stay up. They slide down to my forearm no matter how tight the top and bottom straps are. I quit using them after 3 rides.