ومقام الفصل يباين مقام الوصل
The rhetorical station of separation differs from that of connection.
Fasl leaves two utterances unjoined by a coordinating particle; wasl links them with a suitable conjunction.
جَاءَ زَيْدٌ. جَلَسَ عَمْرٌ. — Zayd arrived. Amr sat down.
جَاءَ زَيْدٌ وَجَلَسَ عَمْرٌ. — Zayd arrived and Amr sat down.
These simplified examples show the forms, not the full classical rules. In ʿilm al-maʿani, the decision depends on the semantic and rhetorical relation between the utterances; the mere presence of waw does not guarantee elegance.