By use of an Extension I spell, the wizard prolongs the duration of a previously cast 1st-, 2nd-, or 3rd-level spell by 50%. Thus, a levitation spell can be made to function 15 minutes/level, a hold person spell made to work for three rounds/level, etc.
Naturally, the spell affects only spells that have durations. This spell must be cast immediately after the spell to be extended, either by the original caster or another wizard.
If a complete round or more elapses, the extension fails and is wasted.