From what I understand, the damage comes from the brain colliding with our skulls. Our brains are floating inside our skulls and a violent collision can cause the brain to rattle around and collide with our skulls, which causes the concussion. Heading a soccer ball may have an impact because it does cause the brain to collide with the skull. However, a collision generated from a 2 pound ball is much less than from a huge guy running full speed. I agree that much of it is sub-concussive.

That said, my concerns with soccer are more related to ankle and knee injuries. Seems like almost everyone eventually blows out a ligament. At least anecdotally.