This exploit involves the daily quests "At The Enemy's Gates", which you get as a valiant, and "Battle Before The Citadel", which you get as a champion. Note that these are the alliance names of the quests and if they are named otherwise on horde it would have been nice if you mentioned this in a comment.
As you probably know, these quests involve the new mounted combat system and in these quest you are fighting four types of scourge which are found north of the Icecrown Citadel.
One of these types can be especially hard, the Boneguard Commander. He has 250 000 HP and starts with three stacks of the shield buff that reduces his damage taken. There are also a whole lot of other mobs nearby and they also respawn at a fast phase. Due to this, you can without noticing suddenly have multiple mobs after you, maybe enough for a certain death.
This is where the exploit comes in. The mobs are in a phased area of the ground north of Icecrown Citadel. If you go southwest like this screenshot indicates:
If you are going in the right direction, you will go into another sub-zone called "The Court of Bones". The border to this goes along some cracks and stones in the terrain if im not mistaken. When going into this sub-zone, you will leave the phased area of the daily quest and the mobs will no longer be chaseing you. You are then free to heal your mount up with the spell because you go out of combat right away.
I know this isnt gamebreaking and most of you probably think the mobs are not too much a problem. On the other hand some of you may so i hope this can help!