أَنْ يَكُونَ بَيْنَ الْجُمْلَتَيْنِ اتِّحَادٌ تَامٌّ
The clauses are so fully united that the second serves as a replacement, explanation, or confirmation of the first.
Where the second clause develops the very same meaning rather than adding a separate report, wāw can make it sound unnecessarily independent. Presenting it directly is more accurate.
Its prominent forms are badal, which restates more specifically, bayan, which removes obscurity, and ta'kid, which reinforces what was already said.