وَالْعَامِلُ فِي الْحَالِ فِعْلٌ أَوْ مَعْنَى فِعْلٍ.
Translation: The governor of ḥāl is either a verb or something carrying the meaning of a verb.
In “Zayd came riding,” jāʾa is an overt verbal governor. In “Zayd is in the house standing,” an understood sense of being located governs the ḥāl.