With a verbal noun, fi can convey a condition tied to an action, because the verbal noun names the occurrence of an event. The book gives a third historical legal formula.
«أنت طالق في دخولك الدار»
You are divorced upon entering the house.
Dukhuluk denotes the act of entering. The text connects the legal effect to that event rather than to an earlier point. Compare: ghad is a time, Makka a place, and dukhul an occurrence.