Getting a sheep off on your opponent can be crucial, either the healer or dps. Without disselecting your target, all you have to do is hold down Shift and it will sheep your focus target. This is very useful for controlling the battle and to lockdown a target.
/cast [modifier:shift, target=focus] Polymorph(Rank 1)
/cast [nomodifier:shift] Polymorph(Rank 1)