Life for Spirit Spent (or LFFS) is an item affix that heals the Monk X amount for each point of Spirit spent. The amount healed is a flat number, not a percentage, and this property rewards Monks with life for spending Spirit via their skills. A player looking to optimize this would want the ability to gain Spirit rapidly, and also to spend it quickly, especially during intense combat.
This modifier exists in D3C but is little used or valued. It is set to become much more prominent in Reaper of Souls as Life Steal is being largely removed from the game and often replaced with LFSS on the Monk skills.
The Health Globe Healing Bonus affix often pairs with and adds to LFSS abilities, making that affix useful to have on gear since it can substantially increase (doubling it or more) the healing bonus from LFSS.
Life for Fury Spent is an identical property used by Barbarians, and it's likely (though not yet confirmed) that the Crusader will have an equivalent property with Life for Faith Spent.
- More reference terms.