I do not believe in the idea of partial communion, but what is a good explanation of this quote of St. Leo I?:
Letter 159 of VII. About those who in fear or by mistake were re-baptized.
"But as to those about whom you thought, beloved, we ought likewise to be consulted who were either forced by fear or led by mistake to repeat their baptism, and now understand that they acted contrary to the ordinances of the Catholic Faith, such moderation must be observed towards them that they be received into full communion with us, but not without the healing of penitence and the imposition of the bishop's hands, the length of the penance (with due regard to moderation) being left to your judgment, as you shall perceive the minds of the penitents to be disposed: in which you must not forget to consider old age, illness, and other risks. For if a man be in so dangerous a case that his life is despaired of, while he is still under penance, he should receive the gracious aid of communion by the priest's tender care."
http://ww.newadvent.org/fathers/3604159.htm
Suggesting that because Pope Leo VII used the words "full communion" means that there is such a thing as "partial communion" is exactly the same thing as suggesting that because the Gospels say Jesus is Mary's "first born son" means that she had a "second born son". In both cases, the conclusions are invalid.
There are a great many things that used to be said that simply cannot be said today because of Modernism. We can no longer even mention "bread" after the Consecration at the Mass because the Modernists have corrupted the concept of the Real Presence so much that the mere use of the term at the wrong time implies disbelief. It is the same in the secular realm where we can no longer say that we are "gαy" because we're happy and joyful.
By the way, if you don't believe in "partial communion" you are committing heresy against the Conciliar Church whose "magisterium" teaches that this is a doctrine of their faith.