A technique employed by the astral elves, allowing spellcasters to work in unison when channelling the weave, synchronising their harmonies for a brief moment to channel the spell together.
Harmonising your casting with another grants a character the ability to apply class features that modify spells to another creature's spell. At least two characters must both have this feat, must have practiced harmonising together and must be within 30' of one another to make use of its benefits.
Characters with this feat gain a new reaction when within 30' of another character with this feat that is casting a spell on their turn. When another character with this feat casts a spell, you can use your reaction to apply either sculpt spell or energy substitution to your ally's spell, if you meet the criteria required for those effects to be applied as if you had cast the spell yourself.