The meaning of Mary's "Preventive redemption"

Pope Benedict XVI said this about Blessed Duns Scotus' contribution to the dogma of the Immaculate Conception (1):

“In the times of Duns Scotus the majority of theologians countered with an objection that seemed insurmountable, the doctrine which holds that Mary Most Holy was exempt from original sin from the very first moment of her conception:  in fact, at first sight the universality of the Redemption brought about by Christ might seem to be jeopardized by such a statement, as though Mary had had no need of Christ or his redemption. Therefore the theologians opposed this thesis. Thus, to enable people to understand this preservation from original sin Duns Scotus developed an argument that was later, in 1854, also to be used by Bl. Pope Pius IX when he solemnly defined the Dogma of the Immaculate Conception of Mary. And this argument is that of "preventive Redemption", according to which the Immaculate Conception is the masterpiece of the Redemption brought about by Christ because the very power of his love and his mediation obtained that the Mother be preserved from original sin. Therefore Mary is totally redeemed by Christ, but already before her conception. Duns Scotus' confreres, the Franciscans, accepted and spread this doctrine enthusiastically and other theologians, often with a solemn oath, strove to defend and perfect it.”

Benedict XVI: General Audience of Wednesday, July 7, 2010.