Concussion

just a random question that if someone can go fairly in depth about what causes the symptoms of a concussion. yes a head injury but what about the injury to the head causes it? I know that when you have a head injury your brain is a bit like if you have something suspended in a liquid and when you shake it, you cause the 'object' (brain) to bash the walls of the 'tub' (skull). but what is the mechanics behind it? like a broken bone is a broken bone ect. just confused on this one mechanic, is it like bruising to the brain cells causing lack of function till repaired?