The creator of the Blender Defender has this to say:

“Have a cat that won’t stay off your counters? I do. I finally got fed up with it enough to do something about it: scare the crap out of him with a motion-detecting blender (while recording the results for my own amusement, of course). “

Hit the website for some hilarious (and informative) examples of the Defender in action. There’s also reasonably detailed instructions on how to make your very own.