جَاءَ الطِّفْلُ يَبْكِي.
The child came crying.
Here yabkī is an affirmative imperfect functioning as a circumstantial expression, indicating that crying accompanied the coming. In the standard analysis this construction does not require—indeed normally excludes—the circumstantial wāw preceding the bare affirmative imperfect.
جَاءَ الطِّفْلُ وَهُوَ يَبْكِي.
The child came while he was crying.
In the second sentence, wāw introduces a nominal clause beginning with huwa. It is not a direct exception to the bare-imperfect rule, because the syntactic construction has changed.