The targeted spell in effect (if cast as a standard action), or a spell currently being cast (if cast as an immediate action) has a chance of being countered. Make an opposed caster level check against the spell's caster. If you win, the spell is ended. If you lose, you cannot attempt to Disenchant that particular instance of that spell ever again. If cast on a magic item, the magic item gets a +15 bonus to its caster level check.