Your quoted one of Schopenhauer's notable conclusions about hand movement doesn't mean he believes we cannot separate cause and effect. In contrary, Schopenhauer believes our knowledge of causality is the exact source of the phenomenal external world according to reference here:
The difference between the approaches of Kant and Schopenhauer was this: Kant simply declared that the empirical content of perception is "given" to us from outside, an expression with which Schopenhauer often expressed his dissatisfaction. He, on the other hand, was occupied with the questions: how do we get this empirical content of perception; how is it possible to comprehend subjective sensations "limited to my skin" as the objective perception of things that lie "outside" of me?
Causality is therefore not an empirical concept drawn from objective perceptions, as Hume had maintained; instead, as Kant had said, objective perception presupposes knowledge of causality. By this intellectual operation, comprehending every effect in our sensory organs as having an external cause, the external world arises.
Schopenhauer considers what the world is beyond the aspect of it that appears to us—that is, the aspect of the world beyond representation, the world considered "in-itself" or "noumena", its inner essence. The very being in-itself of all things, Schopenhauer argues, is will (Wille). The empirical world that appears to us as representation has plurality and is ordered in a spatio-temporal framework. The world as thing in-itself must exist outside the subjective forms of space and time... Human rationality is merely a secondary phenomenon that does not distinguish humanity from the rest of nature at the fundamental, essential level. The advanced cognitive abilities of human beings, Schopenhauer argues, serve the ends of willing—an illogical, directionless, ceaseless striving that condemns the human individual to a life of suffering unredeemed by any final purpose. Schopenhauer's philosophy of the will as the essential reality behind the world as representation is often called metaphysical voluntarism.
So like medieval theological voluntarists such as Duns Scotus and William of Ockham, Schopenhauer considers Will (volition) to be the noumena outside our subjective and intellectual causal nature as the true driving force, also like the Bhakti devotion in Hinduism. And his view on volition influenced later Nietzsche, Wittgenstein, Schrödinger, Einstein, Freud and Carl Jung, etc as mentioned in the same reference.