This rule was introduced to protect a boxer who is taking heavy damage, and even though they have not been knocked down, the referee would break the fighters and begin a count to eight. If the boxer can continue, the fight resumes and they have had a short break from fighting. It is no longer practiced in professional boxing and its merits have been debated. Some argue that it gave an advantage to the receiving boxer, or that it prolongs their beating rather than shortening it.
This is different from the mandatory eight count which is still practiced.
Boxing Unboxed 