Towards the end of the Middle Ages, the rite of Episcopal Consecration in the Western Church had reached a fair degree of complexity. At its heart lay the ancient Consecratory Prayer of the Roman Church. Into this had been interpolated a paragraph from the Missale Francorum. Where in all this was the 'form' of the Sacrament, essential according to the scholastic analysis of sacramental efficacy, to accompany the 'matter', the Imposition of Hands?
In a later century Pope Pius XII was to lay down that one particular sentence within the original Roman Prayer was the the irreducible minimum of a 'form'. The sentence he chose was itself by no means problem-free. It does not actually mention episcopacy, and at its heart is a couple of textual cruces. It does not point with any clarity to what is essential in Episcopacy, as is shown by the fact that in the old Spanish Mozarabic Rite this same sentence appears at the centre of the rite of ordination to the presbyterate.
But in any case, rubricists of the Medieval and Counter-Reformation periods did not look to sentences in nice old prayers for Sacramental 'form'. They liked an 'imperative' form ('Receive ...') or a 'declarative' ('I baptise/absolve ...'); forms which were uttered simultaneously with the 'matter' and which had entered liturgy rather late. (If you don't know the form by which, in the early Roman Sacramentaries, Baptism was conferred, find out. You will get quite a surprise.) So the sentence Accipe Spiritum Sanctum [Receive the Holy Spirit], said as the Consecrators imposed hands on the candidate, became a very popular candidate; indeed, although by the end of the Middle Ages it had not even secured admission to all Pontificals [in England, Exeter had it but the Sarum rubrics do not mention it, although it had probably become customary] by the nineteenth century the consensus of theologians identified the same words in the then current Pontificale Romanum as the 'form' of Episcopal Consecration.
I know what you're wondering. Faced with this complexity and these questions, what deft, sensitive, 'organic' simplifications did Bugnini (the committee was actually chaired by Dom Bernard Botte) perform? Here is the answer: he dumped into his trash-can all three of the formulae I have mentioned; the authentic Old Roman Prayer (which contained the words Pius XII had declared to be the 'form'), the possibly French interpolation, and the medieval Imperative formula (which had previously been regarded as the 'form'). Into the place of all three he shipped a prayer of dubious ('Hippolytan'? No!!!) origin which had been used in the distant Christian East by groups out of communion with Rome whose Chalcedonian orthodoxy was questionable.
Yes, I thought that would make you jump out of your seat.