I'm pretty sure the regressing was shown before the jump just to get across to anyone watching that they're going back to their kids bodies after the jump, and not going back as adults to work along side themselves as kids. more of a clear statement to the audience, than any kind of actual comment on how time travel works in their world. Honestly, I don't expect they'll ever explain the "equation", the idea to me is that it's very very complicated, Five is incredibly smart (smarter than any audience member could be), and is able to, via the equations and careful planning, accomplish certain things, at certain moments. Honestly, it's more of a macguffin to keep Five from coming and going to when and where he pleases at any time he wishes, or else he'd simply be capable of doing anything.