Why can an act of will seem to cause a movement under epiphenomenalism?

Under epiphenomenalism, mental states are considered by-products of brain activity that have no causal power over the physical world. The reason an act of will seems to cause a bodily movement is that both the mental act of willing and the subsequent physical movement are independently caused by the same prior physical states of the body.123

Because the brain state that produces the intention is the same state (or is linked to the state) that triggers the physical action, the two events occur in a consistent, sequential pattern. To the subject experiencing these events, this constant conjunction creates the illusion that the mental act of will is the cause of the movement, even though the mind is actually exercising no influence over the body. In this view, the mental event is merely an effect of physical processes, not a participant in the causal chain that governs physical motion.2314