In prison, when you have access to a big number of fingers, mouths, and legs you can exchange them for things that are worth more to you. What are you gonna do with all these body parts in prison? You should exchange them for cigarettes and toilet alcohol.
You learned this type of exchange from the mighty Genghis Khan (かん). You read a book or watched a show about him once and you're pretty positive that Genghis Khan would exchange the body parts of his enemies all over the continents of Asia and Europe for nicer stuff.