Its healing and damage mitigation. (Recount does not record shield absorption amount)
I have a high holy crit percentage, and high spellpower. My crits proc Divine Aegis, that shields the target for 30% of the heal, also if that shield absorbs the damage, it returns mana from the talent Rapture. So I'm proc'ing shields all over the place, not to mention my PW:S absorbs a hell of a lot more damage then a holy priests PW:S absorbs (mana return works for that too).
If I am tank healing, I can keep stacks of grace on him, and inspiration. Inspiration increases the targets armor by 25% for 15sec after recieving a crit, I also stack grace up to 3x on whoever I'm healing, which reduces damage to the target by 1% and increases the healing recieved by 2%.
My 51 point talent Penance is a channel, which has 3 separate chances to proc DA, Penance heals from anywhere between 10k and 15k HP. Every spell I use is quick and very responsive, I do not have to worry if my target will die while I'm stuck casting a slow Greater Heal. If I need something even quicker, I cast a PW:S on someone, and the talent Borrowed Time Allows me to have 25% haste for my next spell (queue up a PoH, or Gheal).
And with this spec, I only run out of mana if I choose too (DPS for fun & heal).
Basically this spec is about critically healing targets and using/proc'ing shields, preferably on the tank. Excellent for 10mans, currently out of place in 25mans until CoH and WG nerf.