تَعْقِيبُ الْمُسْنَدِ إِلَيْهِ بِضَمِيرِ الْفَصْلِ
Following the musnad ilayh with a separating pronoun.
Taʿqīb here means placing the relevant pronoun after the musnad ilayh. The construction is studied as a rhetorical treatment of the subject, rather than as an unrelated insertion into the sentence.
In «زَيْدٌ هُوَ الْقَائِمُ», «huwa» refers back to Zayd in sense, while also changing how the forthcoming predicate is presented.