Complex sentence: This concave shape shows that the water has acquired a centrifugal form despite the fact that the water is at rest relative to the pail, contrary to the idea that motions can only be relative, i.e., that there is no absolute motion.
Split into simpler sentences: This concave shape shows that the water is rotating, that despite the fact that the water is at rest relative to the pail. In other words, it is not the relative motion of the pail and water that causes concavity of the water, contrary to the idea that motions can only be relative, and that there is no absolute motion.