It seems on Friday night that various people rise at various times towards the end of השכיבנו (and before beginning ושמרו). When is the actual proper moment to rise? (Sources, please.)
In the Nusach Edot HaMizrach...
Right where I put the <> marker is where you rise and accept the extra Neshamah of Shabbat. (You accept the extra Nefesh at the end of Lecha Dodi, and the extra Ruach at Barechu)
(Ben Ish Hai Parshat Vayera #3, Kaf HaHayyim 267:11.)
On Yom Tov it appears that one stands at the beginning of Kaddish, just as one does on weeknights. Even Sepharadim are not required to stand for Kaddish in general, the halacha is that at Arvit, one stands for Shemoneh Esreh at the kaddish immediately preceding Shemoneh Esreh.
If that is the case, this could be the reason why we stand by Ufros Aleinu, to welcome or give respect to the extra Neshama level of the soul (the 3rd level of the soul).