I found a prayer and explanation in a book by Joan Carroll Cruz about it. It is an act of charity of unselfish offering to God all the satisfactory value of one's prayers and good works--plus the value of any that may be offered for you after one's death--for the benefit of the souls in purgatory.
[The explanation says]The "satisfactory value" of a good work is its value with regard for making up for our sins and reducing our stay in purgatory. However the "meritorius value" of our good works is inalienable, i.e. our merits, which give us a right to an increase of glory in heaven, cannot be applied to anyone else. Moreover, a person who has made the act may still pray for himself, friends and other intentions.
Its revocable at will and is not a vow. By making this act with purity of intention, one is relying on the mercy of God and the prayers of the communion of saints to assist his soul after death.
The heroic act was approved and encouraged by Pope Benedict XIII (1724-1730)
And the prayer goes as such:
Oh God, for your greater glory, and to imitate as closely as possible the generous heart of Jesus, my redeeemer, and also to testify my devotion to the blessed virgin Mary, who is also the mother of the souls in purgatory. I place in your hands all my satisfactory works, as well as the fruit of all those which may be offered for my intention after my death, that you may apply them to the souls in purgatory according to your wisdom and good pleasure. Amen.
Thought that was interesting. =)