heroic act of charity
An act by which a member of the Church on earth, in virtue of the doctrine of the Communion of Saints, offers to God (or to the Blessed Virgin), for the benefit of the souls in Purgatory, all the works of satisfaction he will perform during his life, and all the suffrage which will accrue to him after death.
It is commonly regarded not as a vow, but as an offering which can be revoked at will.
Heroism is manifested in the willingness to bear the dreadful pains of Purgatory, that other souls detained there may suffer less, and may sooner enjoy the Beatific Vision.
The Church's approval is attested by the many favors she bestows upon those making it.
New Catholic Dictionary