In Plaut v Spendthrift Farm the court decided that a federal statute that requires federal courts to reopen final judgments that were ordered before the statute's enactment is unconstitutional on separation of powers grounds.
“[O]nce a judicial decision achieves finality, it becomes the last word of the judicial department…. and Congress cannot retroactively command Article III courts to reopen final judgments.”
After appeals are complete and decision is final, the judicial branch had finally spoken and legislative power is no longer effective. Congress may not declare by retroactive legislation that the law applicable to that very case was something other than what the courts said it was. The legislature can affect the outcome of a case by amending the applicable substantive law so long as all direct appeals had not been exhausted.
There is, however, an exception to this.
"Prospective relief under a continuing, executory decree remains subject to alteration due to changes in the underlying law."
“When the intervening statute authorizes or affects the propriety of prospective relief, application of the new provision is not retroactive"
I do not understand what this exception is trying to say. What is prospective relief and what is an executory degree? And what is the propriety of prospective relief?