Yes, its true that its hard to grasp the mechanic. I watched how others played it first time, it was not intuitive..
To control the skull you need to move in opposite direction to pull the skull on the rope. It takes time because of skull's weight and then try to maintain it by slightly moving with WASD (visually you would stay on the same spot).
You don't need mouse to do it. Also the mouse cursor does nothing, it doesn't help to move the skull in any way. Holding left button just makes it easier to start the spin but don't hold it while spinning. (It was a quick hack, because I was tired of tuning box2d params)