Hold the knot at the base (close to the skin) with one hand. With the other, use the comb to pick out the ends first, working inward. If the knot resists for more than 60 seconds—cut it out. A small scissor snip is better than a painful skin tear.
Boxers are notorious for their "witching hour"—that period of hyperactivity, often in the evening, where Oh Knotty Boxer Bits