Oh, um, nothing that big! Most of them were just ordinary Trainers who wanted to help fix things, and they had all sorts of ways to revive the Legendaries. They figured out what was disrupting their cycle of reincarnation, and were able to find various loopholes around that disruption. Like using fossils of previous Legendary incarnations combined with an advanced version of the fossil revival process! Or having a Ghost-type anchor the Legendary's soul.