Healing is a fairy power that is used to treat a wide assortment of injuries. Blue sparks come out of the hands and go into the injury. Healing can also happen if a fairy is hurt and/or unconscious, for the healing power 'goes on automatic'. Healing can be used to reattach parts of a person, as shown by Commander Root using his and Holly's magic to reattach her trigger finger in The Arctic Incident. It can also be used to heal fatal injuries, demonstrated when Holly (unconsciously) saved Butler after his fight with the troll in the first book by repairing his lung, spine, neck and replacing the lost blood in his body. Healing sparks can also be used to burn things, such as when Holly escaped the plastic ties in the fifth book by grazing her wrists to make healing sparks and siphoned off some to melt the plastic.
While all fairies with the ability to heal are able to heal wounds, healing is not an automatic business and when something has been healed, it cannot be undone. For this reason, trickier healing may require a warlock dedicated to this ability.
The process of healing includes replication of cells, which makes it dangerous. In Butler's case it replicated Teflon, Kevlar and ink from a bullet-proof vest stuck in his body. Furthermore, if the wounds prove too much to heal, the spell will drain life force in order to complete. This is apparent in Butler's fatal case, as he ages around 15 years.
It has been noted that Sprites have limited healing powers; enough to magic away a wart, but little more, indicating that aptitude for certain aspects of magic (healing, the mesmer, shielding, etc.) may vary between species.