The two orange/yellow dots represent the centers of gravity for the pistons; the one floating around represents their combined CG. The black/green dot is the axis of the crankshaft. The little black/yellow dot represents the crankshaft's center of gravity. It rotates in the same direction as the red/yellow dot, but is 180 degrees out of phase and thus balances the red/yellow dot.

However, the pistons do not move in pure sinusoidal motion. When the connecting rod is offset, the piston moves at some other speed. It is this difference between the counterweight's circular motion and the near-circular motion of the combined piston center of gravity that makes the yellow/blue dot wiggle a bit. This wiggle is the source of the secondary forces in this design.

Primary balance means that the movements of pistons and counterweights balance each other out. Secondary forces, which must also be balanced, arise from the fact that a piston connected to a crankshaft does not move with pure sinusoidal motion, and the pistons' combined center of gravity does not move in a circle.

One benefit of the way the pistons move is that this engine does not need a large flywheel. The reason is that the two pistons are continuously exchanging kinetic energy. Whenever one piston is stopped, the other one is moving at maximum speed. The rate of one piston's acceleration always precisely matches the rate of the other one's deceleration. The system stores kinetic energy (though because of friction in the bearings, not as efficiently as a simple flywheel would).

More on Primary and Secondary Balance