Horseback archery. The archer shoots arrows at targets as he races past. It is a military art which has been used as a form of divination, bokusen. Famous examples include the ceremonies at Morioka Hachimangu and at the Tsurugaoka Hachimangu in Kamakura.
See Mato-i, O-mato-shinji.
A Popular Dictionary of Shinto. Brian Bocking.