This animation illustrates the volume of one "working chamber" during a full operating cycle. Here, the volume rotation (but not orbit) is nullified, so you may more easily see the expansion and contraction of the chamber volume, and the relative motions of the intake ports and exhaust port. If you have ever watched the sun rise and set, you have seen a similar perspective (the sun is still, as the earth rotates). Notice how the ports rise in the "east" and set in the "west".