Complete connection occurs when the second clause is so closely identified with the first that it functions as reinforcement or elaboration rather than as a separate coordinate assertion. Introducing wāw could wrongly suggest two independently coupled statements.
The reason for faṣl is the opposite of complete disjunction. There, the clauses are too far apart; here, they are too closely joined in meaning. The same visible result—no coordinating wāw—can thus arise from opposite rhetorical causes.