For those that don't know, the buff that you gain from the Ancestral Fortitude talent that increases your max health is called Ancestral Vigor, and I'll be referring to it as AV here. The point of this thread is to hash out what it does and does not do.
Here's a general overview of how it works:
You heal someone in some manner (whether by HR, Earthliving, a Riptide HoT, a direct heal, or a Chain Heal bounce, it doesn't matter, but HST does not apply) and the recipient of the heal gains a buff that increases his max health pool by 10% of the amount you healed him for, and this buff lasts for 15 seconds. Upon receiving another heal (or EL tick, or HR tick, or whatever), the 10% from THAT heal is added on to the previous amount and the buff is refreshed. This continues until the buff caps out at 10% of the target's original (i.e. before AV) max health, at which time further heals simply refresh the buff.
Here are some little nuances of AV that might not be that obvious:
1) It can be stacked via HR on an entire raid before a pull even happens.
2) Multiple Resto Shamans each contribute to the AV health pool, meaning you don't get a bigger benefit from it, but it will stack more quickly.
3) Using HR on cooldown can prevent this buff from ever falling off throughout the entire fight, essentially giving you and anyone else stacked with you a 10% health buff.
4) A simple rolling Riptide is enough to keep the buff on the tank throughout the entire fight, giving them an extra 10% health permanently.
Just thought I'd shed some light on this relatively new mechanic, if anyone else has any good uses for it or tricks you can do with it, post them here.