Extend Spell grants a caster the ability to extend a spell's duration. For 10 more spell points, a spell's duration will be increased by 100%.
The following spells are not affected by this feat (rule-of-thumb: only buffs are extended):
- Protection from Energy (ablative buff spell)
- Protection from Elements (ablative buff spell)
- Stoneskin (ablative buff spell)
- Summoning spells (they all last 10 minutes regardless)
Extend is great for granting teammates long buffs, of 10 minutes or more. If you are about to use a shrine and have spell points left, consider passing out some extended buffs - but only to players not about to rest themselves, because resting dispels buffs. (That technique is particularly important when you are at a shrine to which you will be unable to return). In general, spellcasters should try to use all their SP on something useful before resting, and extended buffs is one way to do that (aside from repair/heal, of course).