how to build: you need a 3' 10" piece of string, feed both ends through the holes and tie the ends in a knot, creating a loop.

to use: put a finger in the loop on each side of the disk. wind up the string by spinning the disk, then pull away from the disk, forcing the disk and sting to unwind. Momentum will wind the disk/string up the other way. repeat. pretty fun once you get the hang of it.