There are two great Powers in the universe concerned with humanity's fate. The first Power long ago laid out the original plan the Earth has been following for millennia -- to an unknown ending. Over time, a rebellious second Power arose that wanted to intervene and make changes. To "improve" things, whatever the risk of throwing the great plan off track. The rebellious Power was more accepting of individual freedom and choice. The two competing Fates are battling for control of humanity.
When a person dies who is on the crux of some change of Fate, someone who could influence things more than usual one way or another, this Power… opens a door. And they offer that person a choice. You can go forward, or you can return and finish your life. If you truly want it, ask for it. Every time Tru Davies saves someone who asks for help, she steers our race's destiny a little further from what was originally planned.
One of Jack Harper's advantages over Tru Davies is that his mentor, Tru's father, is alive, and can fill him in on the long line of knowledge from his predecessors. Tru's mentor should have been her mother, but she died unexpectedly.
The Fates don't give Tru or Jack specific orders. While Tru has no special knowledge other than the fact that she can save people's lives, Jack does does have special knowledge. Not of a future timeline (at least not specifically); but he has access to the knowledge his predecessors have passed down since the first ones got their marching orders. And he had a Near Death Experience with some special moments.
There is a qualitative difference between the power that laid out humanity's grand plan, and the rebellious power that wants to change that plan. The former is more authoritarian in the way it deals with humans, while the latter gives individuals the choice to cooperate or not. Just as the latter offers selected people the opportunity to ask for help, it gave Tru her gift as a gift. Jack, on the other hand, was clinically dead. In his NDE, he came face to face with something he can't entirely remember -- but he was told that if he were returned to life, he'd be working for them.
Once he revived, he wasn't sure what to make of all this. And then his day started rewinding -- whenever Tru's did, though he didn't realize this yet. He ended up in a psych ward. And that's where Tru's father found him. He'd been waiting for someone like Jack to appear, and mental hospitals were a prime hunting ground. When someone showed up talking about re-living days, he knew he'd found his man. So he went to Jack and said, "I know exactly what you're going through -- " (and he could describe it, exactly) "--and I know what happened when you were clinically dead. And I know a lot of other things you'll need to know, so I suggest you listen to me."
Through Tru's father, Jack has access to the knowledge of generations of his predecessors. Tru's mother should have been her mentor, but she died when Tru was too young, and anything she'd written down was confiscated by her husband.