Healing Wish is supposed to heal the incoming Pokemon before hazards damage. So, the hazards damage is applied after the Pokemon is healed. This allows Pokemon like Volcarona with less than 50% health to switch in and recover (and not faint) and end up with 50% health. With all the hazards up and about, this actually makes a big difference in the efficacy of Healing Wish. PO server has this corrected, but not on our server.