Minecraft wiki entry for Wither_cage, associated text: Distracting the wither in a safe way is critical for any wither cage. When the player moves around, it can allow the wither to escape. So, its three heads are often “distracted” with various mobs. Placing a mob like an iron golem within two blocks of the wither’s center head will prevent the launching of the dangerous blue skulls. The center head will look in the direction of that mob, independent of the other two heads.