From Knit Picks website:
Transform hanks or skeins into compact center-pull balls that sit nicely while you knit. A clamp secures the ball winder to the table while you wind skeins up to 100 grams or 3.5 ounces. Best when used in tandem with a Yarn Swift.
Note: The metal yarn guide arm comes folded against the ball winder for shipping. The guide arm needs to be moved and positioned opposite the winding handle for correct operation.