The Sai, also known as the thousand pound bar, has many variations. It was used one handed, as a paired weapon, or in conjunction with another weapon. Sometimes it had a point, rounded tip, or a blade. The prongs also could be rounded, pointed, or bladed. It could have one or two prongs and they could be any size in relation to the center blade/spike. The actual dimensions differed from time period, location and style useing them.
The Sai actually developed wrist strength. While it was a small weapon, the techniques used tend to be at angles the wrist is not accustomed to. It did improve technique leverage, it also caused leverage to be applied in directions and at angles which would never be tried open hand. Since the Sai techniques could be easily countered, the user had to be precise and fast when executing his/her technique. This need for speed had obvious benefits to open hand techniques and, like most things, the power and strength developed allowed more ability to counter those same techniques when the tables were turned. The Sai also is an introduction to the hook concept. What many people don't know is that the Sai's purpose is to break bones and weapons. Most people only joint lock and hit points but you should be able to hook something and break it. If you don't have proper power then you better hope you know pressure points cause the joint locks you are left with won't help you much. obviously you can't train with a partner that way but you should at least be able to break a stick with a joint lock movement. just so you know.