The best technical response I have come across is that by an English Catholic priest and classical scholar (a former Anglican) Father John Hunwicke. He writes an excellent blog: Fr. Hunwicke's Mutual Enrichment.
Fr. Hunwicke wrote:
"I must be both personal and frank. I never doubted that on June 9 1968 I was truly ordained to the Catholic Priesthood [in the Anglican Church] by Harry Carpenter, Lord Bishop of Oxford, and that since then I was truly, morning by morning, confecting and offering the propitiatory Sacrifice of the Lord's Body and Blood. If I had so doubted, I would, of course, have stopped performing what would have been a sacrilegious simulation of so great a Sacrament. I am sure that the five bishops and hundred-or-so priests in the English Ordinariate would say the same . . . .
The great historical fact is that, for hundreds of years, the community of which we are inheritors defined itself in broad, popular, international and cultural terms by opposition to Rome, to priesthood, and to sacramental religion. We helped to torture and to kill those who perceived themselves - and were perceived by others - to be maintaining these things".
Fr. Hunwicke notes that the Bull Apostolicae curae of Pope Leo XIII was a judgement upon the situation as it was in the 1890s. And still in the 1960s this assessment of invalidity formed part of the juridical Magisterium of the Church.
However, since Old Catholics (separated from Rome after Vatican I, but retaining valid bishops and priesthood) participated in Anglican episcopal consecrations from the 1930s, the situation changed in the Anglican Communion generally along with the intention of many bishops to ordain Anglicans into the Catholic priesthood using explicitly sacrificial language and symbolism.
In the case of Bishop Graham Leonard (former Anglican Bishop of London) the CDF in Rome acknowledged this development. They allowed that he should be ordained 'conditionally' to the priesthood (they did not require diaconal ordination in his case). This meant that his orders may well have been valid Catholic orders in the Anglican Church but to put the matter beyond question he was ordained with full Catholic ceremonial to give certainty to the faithful about the validity of his orders.
And so it is obvious that the decision made by the Holy See in his case implicitly applies to all those who are in the same position as Bishop Leonard. In other words, valid bishops with Catholic intent may have validly ordained Anglicans to the Catholic priesthood even if such ordinations are irregular.
Fr. Hunwicke continues:
"If the documentation which he supplied to Rome led the Consultors of the CDF to accept that there was a doubt about the invalidity of his priestly Orders, that is, a doubt about whether the decision of Apostolicae curae still did apply in the changed circumstances of his case, and if the Sovereign Pontiff with his own hand sanctioned this judgement, then, in the mind of the Church, that judgement applies also in principle to those who are in exactly the same position . . . . And when Blessed John Henry Newman entered priestly ministry in full communion with the Holy See, he was uncertain about the invalidity of his orders, but underwent the ceremony required of him, confident that the conditionality of the rite was 'implicit in the mind of the Church'."
So it is that Anglican clergy celebrate the rites of ordination again to provide certainty that their sacramental ministry in the full communion of the Catholic Church is valid beyond question.
You may want to review a previous post here at PEREGRINATIONS written in October 2013 entitled: