Newton's first law of motion: objects in motion (the brain) tend to stay in motion unless acted on by an outside force. The only way to affect this on the field is to limit the force in collisions and/or the number of collisions, especially direct impacts with the head. It really doesn't matter what impacts the head, it could be a shoulder, elbow, knee or even the ground. An impact is an impact. I've read papers that conclude it's really not the big impacts that do the most damage, it's the sum of the lesser impacts (sub-concussive impacts).
Urrgh….must….restrain…urge…to go all…physics-y…
…fail!
Collisions are all about momentum change which is provided by
impulse—the product of force acting over time: F·t. If you want to prevent damage in collisions you either: (a) reduce mass and/or speed to limit the possible momentum change (thus making the overall impulse smaller); or (b) take steps to draw out the
duration of the impact so that a given impulse can be obtained via a smaller force. (Both of these approaches are used to make autos safer in collisions.) The key point is that case (b) always involves
trading distance for time. For example, airbags work by
immediately starting the deceleration process for your head so that you can slow down gradually during the full forward motion toward the windshield, instead of flopping forward freely and then being stopped suddenly by a hard impact with the windshield in the last few milliseconds.
Unfortunately, with football head injuries, there is no space
inside the skull to trade for time—and by the time helmet-to-helmet contact is made, there isn't really much space in the helmet padding to make a difference in stopping the skull, and thus no real difference in stopping the brain
inside the skull.