Check out this YouTube Video showing the basic concept. Do yourself a favor though and only watch the first couple of minutes of the video. God awful music. Sorry to have to send you there.
Like a Spinning Coin
The cylinders actually spin around in a circle sharing spark plugs, it looks a little like how a coin spins on a table. Amazing. The design causes much less vibration, requires many fewer parts, and produces as much power as engines far greater in weight.