Under the Decretals of Pope Gregory IX, the requirements were that either the adulterous pair should promise marriage after the death of the spouse, or that they should attempt marriage while the spouse was still living. While both partners must be aware that they are committing adultery, affected ignorance, ignorance from refusal to investigate what they are doing, does not remove it. Canon law also does not require that the promise be absolute, or even sincere.
If the pair who wish to marry co-operated to kill the spouse of one of them, in order that they might be free to marry, the impediment is invoked, even if they had not committed adultery. Likewise, if one of an adulterous pair killed a spouse (of either partner) in order to marry, the impediment is invoked.
Only the pope may dispense this impediment. There are no instances in which any pope has done so.