An action with this trait requires a degree of mental concentration and discipline.
Manipulate
You must physically manipulate an item or make gestures to use an action with this trait. Creatures without a suitable appendage can’t perform actions with this trait. Manipulate actions often trigger reactions.
Source: Player Core pg. 104
Trigger: An ally within 30 feet Casts a Spell
You use your performance to supplement an ally’s spellcasting, providing magical energy for their spell in their stead. Attempt a Performance check, using a very high DC for the ally’s level (GM Core 53), and either spend a Focus Point (if the triggering spell is a focus spell) or expend a spell slot at least 1 rank higher than the triggering spell. If you succeed at your Performance check, your ally’s spell doesn’t cost the Focus Point or spell slot that ally would normally need to spend.