So, Bob and Charlie are fighting with swords, guns, canes, and Charlie is Disarmed and his weapon clattered away out of reach, Oh no! He's defenceless! or so we believe, for about five seconds.
Charlie suddenly hits Bob with Martial arts, a hidden power or a super technique that he hasn't shown until that moment, and is very good at it. Bob is taken of gaurd at this sudden display of skill.
This trope only works if a character only fights in one style, which is then blocked of, and then changes to another style.