This spell can be used to save a life when low on health and under attack. It is commonly used to get an early first blood by healing oneself for more than one's opponent expects. It is also a skill used for "baiting", where you make your champion appear weaker than it actually is, to lure your enemies out of position or to get them to tower dive you. A similar baiting tactic is also commonly used with Barrier.
This summoner spell is considered by many to be vastly superior to Barrier for a number of reasons - the heal is permanent, affects two people instead of one, gives a 30% speed boost to yourself and your ally, and you cannot heal while at full health (being able to activate the spell requires missing at least one health, whereas Barrier can be completely wasted). However, Barrier is not affected by Healing reduction.
Table of heal strength
Healed champions are debuffed for 35 seconds, causing subsequent Heal casts on them to be 50% less effective for that period.
Heal is affected by any form of personal health gain modifier on yourself (but only yourself), including:
A debuff is now placed on champions healed by Summoner Heal for 25 seconds. Subsequent Summoner Heals will have their effect diminished by 50% on allies with the debuff. Your Champion will still receive the full amount of the heal if cast by your Summoner.
Mender's Faith (mastery) now provides 10 × level bonus health on Summoner Heal from 75 flat bonus health and 10 seconds cooldown reduction.