Minecraft wiki entry for Pre-flattening, associated text: The bottom two bits determine which "side" of the whole portal frame this block is a part of. To make the frame activate, each of the portal frame blocks in the pattern must "face" toward the middle. Since the image is near-symmetrical, it is difficult to tell which direction an individual block is actually facing, but if the block isn't facing in that direction and that is the last frame block where the Eye of Ender is placed, the frame won't activate.