Classically, Sai is used in a pair and held in a position along forearms. The warrior could throw it in the enemy or it also could be used as a substitute for the Sai which had been either broken or lost during the duel. The original purpose of Sai is not known. We also may be surprised at the provenance of this strange metal weapon because in this country the natural stock of metals is extremely poor. There exist a lot of styles of Sai operation.
Methods are similar but there are lots of differences in katas.
The names of schools very often mean the names of Okinawa islands where this or that style had been developed.