You can't stop what happens inside the skull. But the "crumple zone" is designed to slow the speed of the impact thus lessening the concussive force. Kind of like dropping a bowling ball on concrete vs. dropping it on a pillow.
Here's another example.
Air-bags in motor vehiclesAir bags are used in motor vehicles because they are able to reduce the effect of the force experienced by a person during an accident. Air bags extend the time required to stop the momentum of the driver and passenger. During a collision, the motion of the driver and passenger carries them towards the windshield. If they are stopped by a collision with the windshield, it would result in a large force exerted over a short time in order to bring them to a stop. If instead of hitting the windshield, the driver and passenger hit an air bag, then the time of the impact is increased. Increasing the time of the impact results in a decrease in the force.
Therefore if t is increased, for a constant change in momentum, the force on the body is reduced.