PreviousFrameSwitch is used to get the correct motion vectors when using world position offset (so you can have correct motion blur in vertex animation for example). In the material screenshot, it’s telling it the vertex has moved 20 units forward, which makes it have constant motion blur, even when it’s not moving. The first screenshot just shows that in action, and the second screenshot is using compensation for the camera motion. Both are Play-In-Editor.
Maybe I did it wrong, though, so you can still play with it.