[This guide is for Rift.]
Often in raids you will hear the raid leader instruct you to:
focus fire through a certain player
assist a certain player
target through a certain player
All of these can be done in the same way, for they are the same thing. What your raid leader is asking you to do is for everyone in the raid or group to attack the same target. This is easily accomplished. It is also important because you can defeat the desired target much more quickly and complete objectives more efficiently. You will be able to switch targets without ever clicking on anything, because that player (often referred to as "main assist" or "MA") is doing all the selecting and switching for you. In dungeon groups this is usually not the tank, but the primary dps. In Foghladha's event raids it is usually the raid leader, unless your specific raid leader says otherwise.
Here is what you need to do ahead of time to be ready to respond to that leader's orders.
Check "Cast on target's target"
Also Check "Show target of target"
That's all there is to that! Now when your raid leader instructs you to focus fire / target through / assist a certain player, you simply CLICK ON THAT PLAYER in the raid/group window. Now all your attacks will go through that player, and hit the mob that player is targeting, without damaging the player.
If you are a healer, this mechanic also works in reverse. You can target the mob, and heal/buff/cure the player it is attacking. This is an excellent way to heal boss fights. This method also allows you to heal easily outside your raid or group, like at a public rift.
If a raid leader asks you to do this, please do it asap. It benefits everyone in the raid. It will not affect your dps in a negative way.