Law-of-conservation-of-ninjutsu Definition

pronoun

(narratology, fiction, informal, humorous) In a fictional fight, the phenomenon of the team with fewer people often being apparently more powerful than the other team.

Wiktionary

Origin of Law-of-conservation-of-ninjutsu

Taken literally, this law states that each team of ninjas has the same finite amount of "power" of martial arts, therefore a team with less ninjas has more powerful ninjas.

From Wiktionary
law (“rule”) + of + conservation (“preservation”) + of + ninjutsu (“a martial art used by ninjas”)

From Wiktionary