Fa-qṭaʿ is the imperative of qaṭaʿa, with an understood second-person subject. Lubānata is its direct object. The relative expression man taʿarraḍa waṣluhu identifies the person whose connection proves precarious.
In waṣluhu, waṣl is the first term of a genitive construction and -hu the attached possessive. The participles wāṣil and ṣarrām point in opposite directions: apparent joining versus actual cutting.