Every time a boxer receives a punch, potassium rushes out of cells and calcium rushes in, creating an imbalance in the electrolytes, which the brain must work to offset. when the damage is greater thant he brain’s ability to repair the damage, the brain shuts down (knockout). THe feet are a good indication of this happening, as the boxer loses his abilty to stay in balance and so is more flat footed. Usually a knockout punch isn’t one punch, but the accumulation of punches.Source: Popular Mechanics.
A punch to the jaw causes a knockout because the acceleration then sudden decceleration overloads the brain, causing a concusion and the brain to temporarily shut down.