How you can explain in plain English the change of focus (from puthujjana to ariya) that takes place in this stage of insight?
There're a few different interpretations of the word Gotrabhu, which Ven. Bodhi commented to AN 9.10 in his "Numerical Discourses":
"Gotrabhu. In his translation of Vism, where the word is used in a technical sense, Nanamoli renders it “change-of-lineage” (see Vism 672–75, Ppn 22.1–14). Mp explains this person—in accordance with the exegetical scheme of the commentaries—as “one with a mind of powerful insight that has reached the peak, the immediate condition for the path of stream-entry.” Mp is here referring to the gotrabhu mind-moment in the cognitive process (cittavithi) of the path, the mental event that immediately precedes sotapattimaggacitta, the mind-moment of the path of stream-entry. Since this scheme is relatively late and presupposes the Abhidhamma theory of the cognitive process, it is unlikely to reveal the original meaning of gotrabhu. In the Nikayas, the word occurs infrequently. In the present sutta it seems to mean simply a virtuous monk or nun who has not reached the path of stream-entry. We find the plural form at MN 142.8, III 255,6–7: “But in the future, Ananda, there will be clan members, with ochre [robes around] their necks, immoral people, of bad character” (bhavissanti kho pan’ananda, anagatamaddhanam gotrabhuno kasavakAttha dussila papadhamma). In the latter passage it has a pejorative sense, referring to those who show merely the outer marks of a monastic without worthy inner qualities."