وهي كيفية راسخة في النفس
It is a quality firmly established in the person.
Kayfiyya denotes a quality; rasikha marks it as established rather than fleeting. Producing a felicitous phrase by chance is one occurrence. A cultivated ability to do so reliably is a faculty.
A novice might recite a refined passage from memory yet struggle to formulate a new statement. The recited passage may itself be fasih, but that does not automatically show broad, settled expressive competence.