The 661 EVO Knee 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 very comfortable pads, I've been able to ride for hours without any uncomfortable rubbing. I've only had a few "opportunities" to test them out, but they've saved my knees each time. Pretty lightweight, breathe awesome, would recommend them to anyone.