Specifically: after the Elevation of the chalice, what does the celebrant do with his hands and arms as he goes on to the Unde et memores ... ?
According to our present S Pius V Missal, his hands should be extended in front of his chest, palm facing palm. Just as he holds them normally during prayers. (During Mass Practice at Seminary in the mid-sixties, I recall the humorous suggestion being made that the priest was keeping the Sacerdotal Power bouncing back and forth between his two hands ... only during the Hanc igitur did he bring their combined power to bear upon the Elements.)
But in old English Rite of Sarum it was different ... and also in the Dominican Rite and the Carmelite Rite and every other Rite I've specifically inspected, even the non-Roman Ambrosian Rite. Even the Romanised/Jansenised 1846 [is that the version you use?] Rite of Lyons ...
In all of these, he extended his arms in the form of a Cross, thus imitating the Crucified Lord who stretched out his arms for our Salvation upon the Cross. (There is probably a hint in the S Pius V Ritus Servandus that the Priest is not to do this: 'ante pectus'.)
I have no idea when this custom arose ... does any reader? (One can easily guess why.)
And today I am interested particularly in the corresponding question: why was the Rite of S Pius V keen to exclude it, when it seemed so (nearly?) universal?
Very tentatively, all I can think of is this: the Pian rubricists were very anxious to ensure that any fragments of the Most Holy which might have adhered to the fingers of the celebrant should not fall to the ground. (This, of course, is why he is bidden to keep thumb and index fingers conjoined ... and very properly so, too.)
Did it worry people that, with the arms fully extended, fragments might fall which, naturally, would not fall safely on to the Corporal?
English Recusant gentry welcoming the new Seminary priests aftr 1577 must initially have found the New Rite of S Pius V quite strange in its omission of this practice. Moreover, its many genuflexions must also have struck worshippers as strange, since Sarum lacked them.
At least, they are absent from the printed versions of the Sarum Rite. Perhaps they happened in everyday practice, you suggest ... But I think not, because in his 1549 rubric forbidding the Elevation, Cranmer did not think it necessary to forbid the genuflexions (nor did Ridley in his paranoidally comprehensive Visitation Articles of 1550)!
By one of history's delicious paradoxes, the introduction of kneeling before the Consecrated Elements seems to have entered English worship in the Order of Communion of 1548, added to Sarum by Thomas Cranmer, in which the priest kneels for the Humble Access (ditto in 1549). Bishop Stephen Gardener, I seem to recall, alluded to this polemically, and Cranmer responded in 1552 by transferring the Humble Access to before the Institution Narrative.