That's pretty obvious. Your opponent has to cast a heal over time spell. If they do, you have a chance of your pet stealing it for you. It's no different than the steal charm or steal ward. You can't steal something unless the opponent casts it. It's a talent you probably won't see come into play in PvE. In PvP with all the healing pets casting Spritely you'll most likely see it happen there a lot more often.