Jackson can jump into the past just by using his mind. At first he can only go back a few hours, a couple days at the most. As stated before, he cannot change anything, so if he were to kiss a girl in the past she would not remember it in his own time (or home base). If he injures himself, he does not return injured. In fact while he is gone, his home base self remains in a catatonic state. He tells no one about his ability except his friend Adam whose analytical thinking leads them to conduct experiments to test Jackson's ability.
It is the moment when something tragic happens to Jackson's girlfriend Holly that he jumps back farther than he has ever gone - two years in the past to 2007 and finds he cannot return to his own time. He can jump to the past but never beyond 2007. His many trips to his past lead him to suspect his father of living a secret life possibly as a government agent. He also takes the opportunity to visit his twin sister who died of cancer. Time travel is a physical strain for Jackson. It is also mentally taxing to discover so many things about the people around him and not be able to return home to rescue Holly.
I almost gave up on this book. I kept reading when Jackson decided to find Holly in the time before he had even met her. I was intrigued by the idea of Jackson meeting her and interacting with her when she was a younger and a less mature version of the Holly he knows. He also knows so much about her while she is just meeting him for the first time. The old saying is "If I knew then what I know now." That is what kept me reading - that Jackson can approach his relationship with Holly in a different way knowing everything he has learned.
The story has action and lots of twists and never gets to bogged down in some of the more confusing aspects of time travel.