A waterfall is a smooth, continuous flow of water, and shifting like a waterfall indicates that the shift was smooth and continuous.
Consider the statement "And then, as she had predicted, the faeries who were in the lead and many of whom were leopards, shifted smoothly and wondrously into their animal forms.". Is the statement "And then, as she had known they would, three hundred yards from their target the leading faeries, many of whom were leopards, shifted like a waterfall." false? Yes/no/unknown