وَبَدَلُ الْبَعْضِ مِنَ الْكُلِّ: وَهُوَ مَا مَدْلُولُهُ جُزْءُ مَدْلُولِ الْمَتْبُوعِ، نَحْوُ: «ضَرَبْتُ زَيْدًا رَأْسَهُ».
Translation: Part-for-whole badal denotes a component of the antecedent, as in: I struck Zayd—his head.
The head is an actual part of Zayd. The attached pronoun -hu links the part back to its antecedent.