From the website:
The Momote Shiki ritual is carried out on Seijin-no-Hi, Coming of Age Day though the ritual predates the holiday by centuries. Momote - means "hundred hands." The ritual is a bit of Shinto mathematics: ten archers at a time shoot two arrows a piece. The number of archers times the number of arrows equals 100.
The Momote Shiki ceremony is conducted by the Ogasawara-Ryu, one of the oldest schools of Japanese-style archery. The Ogasawara family has long been associated with martial arts training most especially archery. The founder of the Ogasawara line was Nagakiyo who was born in the mid-12th century. He excelled as a mounted archer and was granted the surname of Ogasawara by the Emperor after the name of his birthplace in modern Yamanashi Prefecture.