In RenderMan for Maya, this attribute is enabled by the "Prevent polygon cracking" (rman_preventPolyCracks) check box.
Hover over the image to see its effects.
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"Prevent polygon cracking" should only be enabled when necessary as it uses a bit more memory. The amount used is proportional to the number of vertices in the mesh.
Notice When you hover over the image, notice how using smoothed normals warped the cube's displacement pattern and changed its appearance: