I think he was referring to the fact that overhealing doesn't count towards your threat. So if you have 3 hots ticking while the tank is at full health, then you're not going to draw aggro. The thing that you have to be careful about when doing that is that if your tank takes damage and your hot ticks before he's gotten some threat on all the mobs in a pull, then you're going to draw aggro pretty easily at the beginning of the fight.
@OP:
Regarding your gear, the overcast stuff is better than greens, but a lot of the itemization is wasted on resilience. Concentrate on picking up replacements for those pieces.
Regarding your spec, I'd strongly consider switching to something like
this instead. You'll be far better served by going down to 14 points in balance than buffing tranquility and HT. HT is mainly used with NS to drop a big instant heal to save someone from going down. Generally speaking, you shouldn't be using HT to combat spikes. You'd be better off dropping a regrowth + swiftmend. Just make sure you're using the Swiftmend, Lifebloom, and Regrowth glyphs.
Your basic rotation of rejuv + 3 stack LB on the tank is correct. You should combat spikes with Regrowths and swiftmends. Depending on how much damage other party members are taking will determine whether you should HoT them up, or hit them with a Regrowth. If everyone is taking damage, Wild Growth is your best option. HT should be used rarely, and I'd say only in situations where the tank is taking so much damage that you can't do anything else or the tank is going to die. You really should never find yourself in this situation. If that's the case in heroics, then either you or the tank are significantly undergeared for what you're trying to do.
Tranquility should only be used as a very last resort/wipesaver, and chances are you're going to die anyway. It's on a 10 min CD, so it's not like you can rely on this ability for your generally healing strategy.
Hope that helps.
-Camel