Here is language from the CDC

A concussion is a type of traumatic brain injury—or TBI—caused by a bump, blow, or jolt to the head or by a hit to the body that causes the head and brain to move rapidly back and forth. This sudden movement can cause the brain to bounce around or twist in the skull, creating chemical changes in the brain and sometimes stretching and damaging brain cells.

Concussions Are Serious
Medical providers may describe a concussion as a “mild” brain injury because concussions are usually not life-threatening. Even so, the effects of a concussion can be serious.

(me) I've known of clients injured in a rear end accident that caused them to be concussed. The experts say that when the brain moves around the skull that can and often causes brain injuries because of its impact(s) within the skull. Unless the victim is dying paramedics will first check on orientation, dizzyness, vision, all things related to a concussion,