Interesting Time-Travel Concepts...
They can time-travel to a point before the accident and still have their abilities (see the first episode)meaning they are separate and detached from the timelines being created. Multiple timeline theory.
They time-travel along their own life but it seems the future timelines they enter is based on the point they left. (see first episode) (Quantum Leap Theory)
The apparently can keep jumping back and forth into the same future point and it's a clean slate (see second episode)each time they return to the future point.
They apparently can affect someone else's timeline without totally screwing up theirs (see second episode)(Localized event changes)
Somehow the further future (spoiler) event still happens even after they have made changes in the timeline.