If you tuck your head in, your helmet will absorb any abrasion. I have taken spectacular slides downhill on the road with nothing for than road rash. Brain injury happens due to rapid deceleration, uncommon on moderate speed wipeouts where you skitter across the road. If you hit a wall or tree at high speed, or a car his you at speed, there is nothing you can do. In the case of gravel on the road, the main serious injury is shoulder and wrist, which is often, though not always, avoidable. Hip injuries are even more serious so you should aid to absorb impact on your upper arm, not hip.