Before Purim I did a post touching on one of the fundamental questions about how shlichus operates: If Reuvain appoints Shimon to be his shliach, do we view Shimon as if he legally has become Reuvain, or is Shimon still Shimon, but his actions count as if done by Reuvain? This chakira is used by the Ketzos to explain the dispute between the Rambam and the Tur regarding a husband who appoints a shliach to deliver a get and then goes insane. If the shliach’s appointment causes him to legally become identified as the husband, then the mental status of the “real” husband is no longer relevant. If the shliach’s retains his own legal identity and his actions are just ascribed to the “real” husband, then the real husband’s state of insanity would invalidate the get. (Just for the record: everyone agrees in such a case that the get is invalid; the issue is whether the psul is d’oraysa or derabbanan).
My son started learning the second perek of Kiddushin by himself and wanted to use this chakira to explain the debate (43a) over whether a shliach can serve as a witness (“shliach na’aseh eid”). If Shimon the shliach assumes the identity of Reuvain, the one who appointed him, then Shimon becomes a party to events and cannot testify about them. If, however, Shimon legally remains Shimon and his actions are just ascribed to Reuvain, because he retains his independent identity he is not precluded from testifying. (The same analysis can be found in R’ Yosef Engel’s sefer Lekach Tov).