how does deep healing work in wow?

Answer 1

Increases the potency of your direct healing spells by up to 24%, based on the current health level of your target (lower health targets are healed for more). Each point of Mastery increases direct heals by up to an additional 3.0%.

Answer 2

Healing rain got a stealth buff.
Tooltip reads ~1100 healing per 2 seconds.
Actual heal on a single target 2300-2400
Diminished heal on 8 targets 1750-1900 up from ~1200 last week BEFORE that patch but after the hotfix.
Dimished heal on 25 players+some pets 700-900 up from ~300 last week before that patch after the hotfix.
Every tick can proc ancestral healing and earthliving weapon.

I'm currently sitting at 39% deep healing, I didn't really do anything with my resto gear but ended up having to heal so put anything availible in places.
Other shaman was sitting at ~53% deep healing, his healing rain was critting for 6k on a group of melee.