Does the brain have to hit the inside of the skull to cause a concussion? or can it just accelerate quickly without hitting the inner skull?

In real life...: In real life, the skull is there, and the brain contacts the skull during rapid acceleration or rapid deceleration. There is no other way, in the real world. For example, suppose a person is standing in line, a very tight line where each person is only 1 inch away from the next person in line. There would be no way to rapidly accelerate one person without bumping into the next person.