This tool is used to cut flexible copper pipe. The copper is soft, so if you were to just cut it the pipe would squeeze shut. Instead you clamp down on the pipe with this crazy tool and go around and around, cinching the tool tighter each time until it breaks the copper. There is a little blade that comes out the side that is used to clean out the inside of your cut. Its pretty neat.
The other day at the hardware store the guy helping me was joking around about how this tool is one of those things you would see in your grandfather's shop and never know what it was for. I love the magazine Cooks Illustrated, so it remind me of the section where they have a reader submit a kitchen tool that they have no idea what it is for... classic "I found this in my grandmother's kitchen tools, but I cannot figure out what it is meant to do...".
So here it is, a copper flexible pipe cutter.
I don't know what I would have thought this was if I found it in my fathers workshop as a kid... perhaps a finger cutter-offer :) eek!