This happened at night when Carter was driving with Alex, Claire, and Billy in the car. And when Alex looked at his seat belt, the belt was broken in half. Then at the tracks, Alex looked down to take off his seat belt and it was put together again.
That would be his foresight into how to save the guy belted into the car. He saw a bus in the window, he could've saved the girl from the bus. He sees the way to save people from death's plan.
In other words, break the kid's seat-belt... save the kid.
That's because Alex was having a vision. He was seeing how he was going to be able to intervene with Carters death. The seat belt wasn't actually ripped.