I am working on a tank, and can easily do story line, but I need to understand how, and what addons might help, to do the aggro switch. I know the aggroing tank gets a stack on them that drops to zero, but that the other tank is supposed to get the aggro when the other tanks is 10 or so, but how do you know, as the other tank, when to taunt and get aggro back (ive got the aggro gadget)? And does the tank who has the aggro and needs to drop, just back off for a while?
How can you see the aggro stacks on the other tank? Do people use add ons?
There probably is some beginners guide that explains this but I cant find it.
Thanks
The other tank needs to be selected as your defensive target (Ctrl-mousewheel through raid group, for example). That way, you’ll see key info about them, including the countdown stacks.
Select the other tank as defensive target so you can watch his stacks, usually you switch at 10 stacks or so.
A VERY usefull addon is Effect UI, and you should grab it for many other purposes as a tank (keeping track of your CDs ? watching for casts you need to interrupt ?) and for any other role too. It lets you see big and clear in the middle of your screen whatever buff/debuff/cast you want to see. You can customize size and color and set special allarms …
This is because raging volcano wasn’t changed like the other taunts when the raid was added, so it’s taunt is still only temporary. So even if the other tank does nothing at all the boss will likely switch back once volcano ends.
This isn’t necessarily a dealbreaker though, so as long as the other tank doesn’t need a break to recover cooldowns or anything. 
I heavily recommend Five Alive mod. Aside from being brilliant for nyr, it generally makes the whole group UI a lot nicer to look at, especially for raids. Five Alive displays the more important info on all your group members over in the group list, so you don’t have to def target anyone. It is indicated with a red mark and a number (of stacks) in white on top of it.
Generally you switch at around 10 stacks. It can be a good idea to stop using power abilities when your own stacks it 10, to help the switch to stabalize.
Also worth to note that whoever has the aggro will get an increasingly darker screen, the lower the stacks get. This is a good way to tell if you or someone else got aggro.
Thanks all. Good suggestions.
You can turn buff tracking on for the entire group and watch their stacks via just the raid bars too (I used that a lot as heals to see who needed cleanses in dungeons without having to scroll through everyone)