My chinese isn't great so it was hard to follow the storyline, so I might've missed something, but where did he get the ability to travel backwards in time?
Remember when he was a baby the man who found him near the basketball court brought him to the martial art school? Besides handing baby Jay to Jay's future sifu, he also gave that guy a book. That book is a martial art manual and it contains an art that can move time. That's the art Jay used to turn back time.