PISCATAWAY, N.J.—When the guy swung at Mark Donnelly, his only means of defense was a black umbrella—and a foppish one at that. But, ducking under a roundhouse punch, he jabbed the pointy end of the umbrella into the attacker's gut, stopping him cold.

Mr. Donnelly, who is 43 years old and several inches short of 6 feet tall, then straightened his waistcoat, and the two men shook hands.

The...