Spell Lock

One of the most powerful uses for this spell is that it is able to use while the warlock is silenced or stunned and the felhunter is not.

Useful in two on two for shutting down a healer while your team mate bursts down the rogue on you...

One of the reasons that Felhunters are excellent pets in arena.
This, when combined with Curse of Tongues and Drain mana are excellent ways to slow down and eventually stop all casting by another player.

Note: The spell is set to autocast. Your pet will automatically cast it on the first mana user it attacks, wasting it. My advice is to bind it to a key so that you have the silence ready for when you need it. (And take it off of autocast!)

From 5.0.4, when the warlock sacrifices a Felhunter (with the talent Grimoire of Sacrifice ), he gets this ability for him/herself. Search for Command demon in your spellbook to use it.