I can't get a photo of it right now, but I mean the girl riding will actually flip upside-down or rotate 90° as the animation plays out. Will update in a bit when I'm at my computer.
Can't really upload this to youtube so you'll have to believe me. The shaking only occurs when the person with the penis has their head to the left side. On the far left of the image you can see the size I'm working with. To the right are the two angles that she jitters between during climax. There is a certain size larger than this where you get a full 180° rotation, and then after that it fixes itself.
