Some animation. http://pinnacle-engines.com/technology.html
The basic concept of an opposed-piston engine is so simple that it’s remarkable how long automakers have been ignoring it. In a traditional engine, each piston sits in its own cylinder, and combustion occurs inside the cylinder head, where intake and exhaust valves regulate the inflow of air and vaporized fuel and the outflow of exhaust. In an opposed-piston engine, there’s no cylinder head: two pistons move toward and away from one another inside the same cylinder, with combustion occurring in the in-between space at the moment of greatest compression. Without the cylinder heads, the engine is lighter, and the dual-piston action means compression ratios are higher, so it gets more bang for the buck.