Minecraft wiki entry for Wither_cage, associated text: If the player wants the black (or blue) skulls to be launched, a mob must be placed between 3 and 10 blocks away. This is in the range to allow for skulls to be launched. Mixing the two techniques above gets a mob near the wither to distract the blue-skull-launching center head, and a mob farther away to make the black skulls launch. The tricky part is ensuring that line of sight is maintained, while preventing the skulls from hitting the mob it is targeting.