I’ve been lead to believe: standing on them is to stress the new hubs in a way which produces the same effects as riding – the hard steel spokes bite into the softer alu hubs – you then do another tensioning session afterwards to get their final tension.
To de-twist the spokes you bring one side of the wheel into you abdomen, holding the opposite side with both hands and then, using your elbows, apply almighty pressure to try and turn them into the shape of Pringle – aka a hyperbolic paraboloid. This momentarily decreases the spoke tension enough to allow them to de-twist all by themselves.