Sai are typically utilized in sets of three (one in each hand and one in reserve for throwing purposes). Contrary to what many think, in Kobudo (the Okinawan weapon style) the sai are often held with the long metal prong along the forearm to be used in blocking and other defensive techniques. The two shorter prongs on either side of the main shaft are used for trapping (and sometimes breaking) other weapons such as a sword or bo.
In Okinawa the sai was used by domestic police to arrest criminals and for crowd control, the use of the sai was perfected in 1668 by Moto Chohei, an Okinawan prince.