The website of the Order has now been updated to include an English translation of the letter from the Apostolic Peniteniary, granting the plenary indulgence for the Jubilee celebration of the 8th Centenary of the founding of the Order of Preachers.
You can find the link here.
It appears as though the indulgence can be gained for the poor souls in Purgatory in two (2) ways. By uniting themselves in their hearts with the spiritual purpose of the Holy Year of Mercy:
- While devoutly participating in the Jubilee Year celebrations.
- In the churches and places of worship of the Dominican family which they will visit on pilgrimage, and where they meditate before the Blessed Sacrament for a period of time, concluding with the Lord's Prayer, he Creed, the Jubilee prayer, and the invocation of the Blessed Virgin Mary of he Most Holy Rosary.