Shaft passer
This image [1] explains the mechanism better. Next, imagine the cord is in fact the shaft of another pair of wheels.<p>[1]<a href="http://drgoulu.com/wp-content/uploads/HLIC/849e60a7d99c30c2af0480c7c5b49d71.jpg" rel="nofollow">http://drgoulu.com/wp-content/uploads/HLIC/849e60a7d99c30c2a...</a>