Useful only to small droplets: basically the vapor barrier reduces the frictional coefficient (or the surface energy depending on how you look at it).
Still must overcome gravitational body forces, and their "trick" does so by utilizing random motion in a heated droplet to take advantage of the momentary shift in the center of mass. Once the center of mass goes "over the ledge" the droplet moves.
Were the droplet to touch the top or side of a channel, this wouldn't work so well.....