Sai were not indigenous to the Ryukyu Islands. They were probably imported from China via the Fukien trade route. It is probable that in the search for arms by government officials, they did not seem like weapons among farming and fishing tools. They weapon itself has many names and is common throughout the Far East. Sai were used in pairs and may have been introduced to Okinawa as weapons of Chinese martial arts, then later taken and adapted by the Ryukyu styles.
The shaft and tines of the sai were used to block, strike and ensnare enemy weapons. Sometimes an extra sai was carried in the belt as a backup. The shaft of the weapon was never bladed, which would have made any reversing action in order to strike with the hilt, or block with the shaft, along the forearm, completely impossible. If the sai were used for thrusting to any depth into the enemy's body, because of suction it would be very difficult to withdraw the weapon, which would render it useless. This may be another reason why an extra sai was carried. Many of the weapons kata from the Ryukyu Islands were named after their founder, or their place of origin, and have been passed down to modern times.
The Okinawan Karate Club of Dallas teaches Toei Sai (also called Chatenyara Sai) and Towata Sai as taught to us by Shimabukuro O'Sensei. We also practice Hamahiga Sai as taught to us by Christensen Sensei.
-Michael Finn, Martial Arts: A Complete Illustrated History, The Overlook Press, 1988