Hiro’s reference is the Ninjutsu

Ninjutsu (忍術), sometimes used interchangeably with the modern term ninpō (忍法), is the strategy and tactics of unconventional warfare, guerrilla warfare and espionage purportedly practiced by the ninja. Ninjutsu was a separate doctrine in some traditional Japanese schools, which integrated study of more conventional martial arts (taijutsu) along with shurikenjutsu, kenjutsu, sōjutsu, bōjutsu and others.

It is believed that the secret clans of the ninja emerged in Japan simultaneously with the samurai class in the IX-XII centuries. The ninja is considered to be the prototype of the yamabushi sect of mountain hermits from the 8th century, who practiced the Shingon school of Buddhism. Yamabushi encouraged hard and grueling training of their adepts, as well as carefully preserved knowledge of medicine, chemistry, astrology gathered over the centuries. The Yamabushi were proponents of complete fusion with nature. Initially, the secret schools of ninjutsu had nothing in common with military organizations, neither in their training methods nor in their philosophy. Over time, however, a change occurred.

Traditionally, ninja performed the role of mercenaries who offered themselves as spies, assassins, saboteurs, political provocateurs to large and small rulers of the Japanese feudal era. Ninja offered their services only to those willing to pay for them. Ninja families were tight-knit collectives integrated into larger groups - clans. It was virtually impossible for an outsider to join such a group and become a ninja; you had to be born into a professional family to do so. The art and other tricks were kept secret and passed down only from father to son. Revealing the secret meant death by the hand of a ninja of the same clan.