Why does a concussion show no symptoms right away?

Definition: The classical definition of a cerebral concussion is that of a brain injury causing a transient loss of consciousness with complete recovery. It is usual not to have symptoms. Many things are called concussions but are not truly classic in their presentation.

Not true: Many concussions do present with symptoms right after the impact, such as football players stumbling/getting confused/possibly even losing consciousness directly after a big hit.

To clarify, a concussion causes loss of conciousness, or amnesia, and/or both. With amnesia and resumption of consciousness, the awake individual was not aware of what happened, and may well deny it. In absence of independent observation, true events can become obscured, so belief of "no symptoms" is indeed a mislead concept.