From a Machian perspective, in an empty universe there is no conception of spinning, so the water’s surface would always be flat (or, to avoid issues of the lack of gravity pulling on the water, we can say that the tension on the rope tied between two rocks will always be slack). The statement here is that, by contrast, in special relativity there is a notion of spinning, even in an empty universe, so that the water’s surface can be concave (and the tension on the rope tied between the rocks can be taut). In this sense, special relativity violates Mach’s ideas.
