Ah, cornstarch and water. So humble, yet so non-Newtonian. You may remember this video (3rd down) of the great fun that can be had with a swimming pool of cornstarch solution.
But what happens when you combine cornstarch with a vibrator, you ask? I have a video to show you! It gets pretty freaky at the end with all the Fardaday waves and undulating fingers:
Although I have no expertise in understanding how all this works, or why it occurs, my understanding is that it is essentially caused by cornstarch's properties to act as both a liquid and a solid, becoming less liquidy and more solid as greater stress is applied to it. That's what allows you to run across a swimming pool of it, but to swim gently in it as well.