The kusari fundo, sometimes also known as the manriki gusari, is a traditional Japanese training tool that consist of a length of chain with a weight on each end.
Developed as a way to deal with armed samurai, it was also often employed with the jutte (iron truncheon).
Naturally in our own training a safe substitute is used made out of padded cord.
Training begins with learning how to manipulate the chain and weights as a way to control distance- extending the "reach" of the person, while using the chain to entangle limps, and immobilize parts of the body.
Additionally the weights are applied to various kyusho (weak points) on the body in order to break structure.