It is just what it's called. It's a ButtKicker. Better known as a "Bass Shaker".
You mount it under your Throne, and it's powered with a Power amp, just like a Subwoofer, and when you hit your Kick drum, it jars your balls loose.
One of the drawbacks of playing electric drums, or triggering, is that there's not a lot of low end from the kick drum (or none at all if you are playing on an electric kick drum pad) so this thing gives you the FEEL of a Kick drum in your body.
They are killer if you mount them in a Couch or Chair and use them for Home theater...you get the feeling of low end, without having to actually "hear" it.
Hardcore Gamers use them as well, and they build "game chairs" with them - but they just use the smaller "bass shaker" brand.
Tim