useBlobShadow puts a soft dark disc on the ground under something and keeps it there. It is the cheap shadow the target consoles actually used: not a shadow of the shape but a smudge under it, and not a feature of the renderer at all: a useCircleGeometry disc wearing a mesh3d material that fades from the middle, drawn down the ordinary model path. No pass, no texture, nothing reserved.
Both cubes hop identically; only the left one's shadow follows the height, and covering the shadows makes the two indistinguishable. The disc also opts out of casting a real shadow, so a scene that later turns on shadow mapping does not get a dark ring nothing explains.