The manriki gusari was a weapon which was a weighted chain, about 3 feet long. It was originally made for self-defense, but in the hands of someone skilled it could be a dangerous weapon. It was famous for being used by ninja because the weapon could easily be concealed in the palm of your hand. Since it was a chain, it could also be used for other purposes, and partially because of that, it was not illegal. (The hinin were not allowed to keep weapons, but it wasn't technically a weapon, so...)
It was used by holding one end of the chain and swinging the other weighted end around like a whip. It could also be used to wrap around the legs of an opponent or to entangle their sword. Of course no self-respecting samurai would be caught using a hinin weapon. (Take that with any amount of sarcasm necessary, who am I to judge the honor of figures in ancient history?)