When cast, Azuth's mantle created an aura that protected a single individual from two specific spells or spell-like effects. It absorbed the effects of the spell, or magical item discharge, then allowed the recipient of its effects to make a choice. The caster could use the absorbed magical energy to heal the mantle-bearer, or launch a different, specific spell towards another individual.
The spell from which the recipient was protected, along with the spell that was "reflected", were both chosen by caster during the casting of Azuth's mantle. The latter of these spells had to be of equal or lesser power than the former.
Azuth's alteration mantle had to be cast on a willing recipient or individual that was taken by surprise.