Its the strength bar.
When you do races Properly. The driver slows down as his strength bar decreases.
This means his Lap time increases.
IF you are playing correctly Your driver will then pit at the strength bar level you select and swap with a fresh driver. (go faster)
IF your are just over powering your way to the end of the game.
You just leave the Pit stop bar at off and the driver will keep going till ether the tires are knackered or will run out of fuel before the end of the lap, so will pit to avoid both.
If the strength bar is low he can just miss his pit stop completely due to tiredness and drive nearly 1 lap at 50MPH. But if your overpowering the car for that event, even that in mostr cases wont lose him the race.
Have fun
