The caster created an area in which only good creatures could be magically summoned. No matter what a later caster attempted to summon, he or she actually summoned a good version (a celestial beast rather than a fiendish one, for example) or a good equivalent (an archon rather than a formian).
If the summoner was attempting to summon a neutral or evil creature, the good creature that appeared was not bound to obey the commands of the summoner or attack his or her enemies. Instead, it dealt with the summoner as it saw fit — possibly attacking an evil caster. The summoning spell could not be dismissed, though it could be dispelled normally.