You’ve learned to cast a particular spell without magical gestures.
Regardless of skill level, you never have to have hands or feet free to cast.
This doesn’t affect effective skill level, FP cost, or casting time.
You must specialize by spell.
Once you know the chosen spell at level 15+ (20+ in low mana), gestures are optional anyway (see p. B237).
At that stage, your concentration is so profound that you get +3 to the Will-3 roll to avoid distraction while casting the spell.
Mages with the limitation Extravagant Rituals (Thaumatology, p. 24) can never take this perk.