Kancho's essay Back NO,9 
The shape of "SAI" looks like the dagger.Therefore, the Karate-Ka to teach the use of "the SAI" like the dagger are not few. It is a very dangerous misunderstanding.The opposing force of The SAI is always "The BOU" (about 180㎝, 900g, and oaks). KATA of The SAI and the technology are made on that condition. The leading part of SAI is "Guard(Tsuba). "SAI is a tool to twine DOU by essentially using the "Guard" and to capture it.The SAI is the policeman's weapon in the Ryukyu kingdom age. The essence of the combat method of the SAI is to lock the BOU, and to suppress the enemy.

This defense to the BOU attack is improper.This is dangerous and improper defense.

The BOU will pass between the SAI, and attack your head easily.

The correct defense to the BOU attack ."The Wing(Tsubasa) " of SAI engages with a strong way of the cogwheel, and the movement of the BOU is sealed off.In this body pose, the BOU cannot be pushed, and or cannot be pulled.
The chance is seen, and the enemy is pierced with Sai.

The Wing( Tsubasa) is a main body of the function of SAI. It is necessary to learn the method of suppressing the enemy by hanging enemy's BOU with"Wing" of SAI.。