A character that has been the target of a spell sometimes carries permanent residual effects, leaving an echo of the original spell. Beneficial spells can linger on a character like a mystical force, bestowing an advantage somehow related to the original spell. In contrast, some PCs have a defensive reaction to hostile spells—in suffering the effects of a spell, they develop a countermeasure.
Such spell-touched characters are a mystery to academic-minded spellcasters, who can’t reliably duplicate the process by repeatedly casting the same spell on a subject.
Accordingly, the only way to become eligible to select a spell-touched feat is to have been exposed to (that is, targeted by or otherwise affected by) one of the spells associated with the feat. If the spell allows a save, you must have failed a saving throw against it at least once, whether intentionally or not. After meeting the prerequisite, you may select a spell-touched feat when your character would otherwise qualify for a feat.