Jack's eyes stayed on the truck. Which direction they went next would depend on which direction that truck decided to go.
Jack saw the truck make a U-turn and back to where it came from. It probably wanted to circle back to the street on the other side of this alley to resume its chase. Continuing forward might have a left turn to that street as well, or there might not be a left turn. The truck driver didn't want to take the risk, so he did the U-turn.
Seeing that, Jack told Stefan, "Turn right after coming out of the alley!"
This time, Stefan didn't argue. He followed the instruction.
They came out to the street they previously were. By turning right, they were going in the direction they came from. Since the obstacles had been cleared by the truck when they passed this street earlier, Stefan could just go straight at high speed. This effectively put more distance between them and the truck.