They use new functions and events entirely.
https://github.com/Ketho/wow-ui-sour...etBattleUI.lua
Code:
PET_BATTLE_PET_CHANGED
PET_BATTLE_MAX_HEALTH_CHANGED
PET_BATTLE_HEALTH_CHANGED
Code:
C_PetBattles.GetHealth(petOwner, petIndex)
petOwner and petIndex are table entries of the unit frame in question, e.g. PetBattleFrame.BottomFrame.PetSelectionFrame.Pet1.