A decree of the Sacred Congregation of Indulgences dated 18 December, 1885, and confirmed the following day by Leo XIII, says:
The Heroic Act of Charity in favour of the souls detained in purgatory consists in this, that a member of the Church militant (Christifidelis), either using a set formula or simply by an act of his will, offers to God for the souls in purgatory all the satisfactory works which he will perform during his lifetime, and also all the suffrages which may accrue to him after his death. Many Christians devoted to the B.V. Mary, acting on the advice of the Theatine Regular Cleric Father Gaspar Olider, of blessed memory, make it a practice to deposit the said merits and suffrages as it were into the hands of the Bl. Virgin that she may distribute these favours to the souls in purgatory according to her own merciful pleasure.
Olider lived at the beginning of the eigtheenth century.
The Heroic Act is often called a vow, yet it partakes more of the nature of an offering made to God and to Mary, and it is also, unlike a vow, revocable at will. This point has been decided by the S.C. Indulg., 20 Feb., 1907, in answer to a question from Chicoutimi in Canada. A special vow "never to revoke the Act" would probably be binding, because its subject matter is an act of the personal will of which man can freely dispose, whereas he has not the disposal of his satisfactory works in favour of the departed; that depends on God; for man it is only a matter of pious desire, and only in this sense a votum. It always remains doubtful to what extent God accepts the oblation, and it is certain that the holy souls altogether lack the power of accepting it. The practice of the Heroic Act is based on the communion of saints, in virtue of which the good deeds of one member of Christ's body benefit all other members. Its meritoriousness results from the more intense charity (love of God and His suffering friends) which inspires it, and on which the intrinsic perfection of all our good deeds depends. Its heroicity arises from the willingness it involves to take upon one's self the dreadful pains of purgatory for the love of one's neighbour, although there remains the reasonable hope that God in His goodness, and the sainted souls in their gratitude, will not allow the punishment to be exacted to the full.
The Heroic Act has been enriched with numerous indulgences by Benedict XIII (1728), Pius VI (1788), and Pius IX (1852). Priests who make it receive the personal privilege of gaining a plenary indulgence for a soul of their choice each time they say Mass (see ALTAR, under Privileged Altar). Laymen gain a similar indulgence each time they receive Holy Communion, also each Monday they hear Mass for the departed; in both cases the usual visit to a church and prayers for the intention of the pope are required.