Minecraft wiki entry for Wither_cage, associated text: Before the wither is spawned, a piston needs to be placed facing down, above the “tail” of the wither spawning formation, with one block of space between the soul sand and the piston. Summon the wither, and then power the piston with a button. As the piston retracts, place obsidian in the space where the piston arm was. This must be done quickly, or the wither might float upwards and escape.