If I recall correctly, in this design the engine has 2 cylinders (2 pistons each...

If I recall correctly, in this design the engine has 2 cylinders (2 pistons each) and each piston is linked to the opposite piston on the other cylinder. In older designs you just have 2 pistons per cylinder but no cross-linked pistons.

Compare this: http://en.wikipedia.org/wiki/File:Opposite_piston_engine.gif

with the pic in the article.