Arm Whip is described with Waterwheel Drop on page 151 of Ring Dream.
It is easier to understand by seeing it in action, such as this example with Chris Hero.
It appears to involve an arm making a Grapple against the opposite arm of an opponent standing on that arm's side who is facing the same direction, and then performing a Judo Throw and they flip over in the process of Falling into a lying position, supine onto their backs.