This weapon is used in many modern styles of martial arts, although this weapon is rarely seen in SCA combat because it doesn't have any particular function in that style of fighting. The Sai is a fork-like weapon, about 15 - 20 inches (40 - 50cm) long, usually weighing about 3lb/1.3kg. It's believed to have been a farmer's tool used for planting, somewhat like a pitchfork, and has documented use in Okinawa, India, China, Malaysia, and Indonesia as a weapon.
It has a shaft tapered to a point from a blunt end, with two tines (aka 'tangs') either side protruding forward about a quater of the distance from the end of the shaft. It's considered to be a defensive weapon against the bo and sword which can be hooked between the tines, but it can be used to stab, strike or throw. Two sai are carried (one in each hand) with a third in the belt to be used in case another is thrown.
Just like with use of an SCA sword, power is generated by loosening and tightening the grip of the fingers, with the thumb and forefinger controlling the shaft.