Good question. Here's a quick answer from the EcoMotors website:
"The engine operates on the 2-cycle principle, generating one power stroke per crank revolution per cylinder. It comprises two opposing cylinders per module, with a crankshaft between them, and each cylinder has two pistons moving in opposite directions. This innovative design configuration eliminates the cylinder-head and valve-train components of conventional engines, offering an efficient, compact and simple core engine structure. The result is an engine family that is lighter, more efficient and economical, with lower exhaust emissions compared with conventional designs."

There's also an explanation video with professor Peter Hofbrauer, who invented the engine, embedded after the jump. If any mechanical engineers want to chime in with further enlightenment, two cents or what have you, lay down some knowledge in the comments section.