Complex sentence: An engine is used to counter-rotate the cylinders--one cylinder turns clockwise and the other counter-clockwise, the counter-rotations cancel out so that the vehicle moves forwards (or backwards) along the axis of rotation.
Split into simpler sentences: An engine is used to counter-rotate the cylinders--one cylinder turns clockwise and the other counter-clockwise. The counter-rotations cancel out so that the vehicle moves forwards (or backwards) along the axis of rotation.