When you right click on an enderman with Shackles, it will become an enderslave. The ender slave can be right clicked on for a menu to appear, which will allow you to change its mode. An ender slave will never despawn, until killed.
Relax mode, is the default mode, it will make the enderslave stay still until you change the mode.
This will make the enderslave follow you, and defend you.
This will make the enderman pick up any nearby valuables (coal, iron, redstone, gold, lapis Lazuli and diamonds) and take them to you. This is useful for if you often miss ores.