دَخَلَ الْوَلَدُ يَضْحَكُ.
The boy entered laughing.
Here yaḍḥaku is a bare affirmative imperfect describing the boy's state. The classical rule ordinarily excludes a directly prefixed circumstantial wāw with that bare form.
دَخَلَ الْوَلَدُ وَهُوَ يَضْحَكُ.
The boy entered while he was laughing.
The pronoun huwa introduces a nominal structure containing a verbal predicate. This is not the same construction as adding wāw directly to the bare imperfect; distinguishing clause structure resolves the apparent exception.