A more thorough explanation:

The sudden-onset rule is a principle in law that states that medical testimony is not necessary to prove the cause of obvious symptoms that immediately follow a known traumatic incident.

For example, if someone is involved in a car accident and immediately experiences severe back pain, the sudden-onset rule would apply. The cause of the back pain is obvious and does not require medical testimony to prove that it was caused by the car accident.

Another example would be if someone slips and falls on a wet floor and immediately experiences a broken arm. The sudden-onset rule would apply in this case as well, as the cause of the broken arm is obvious and does not require medical testimony to prove that it was caused by the fall.

The sudden-onset rule is important because it allows for quicker and more efficient resolution of cases where the cause of the injury is obvious. It also helps to prevent unnecessary delays and expenses associated with obtaining medical testimony.