It's a setting which automatically tells the driver to pit in when his physical strength bar (the green one) is at a certain percentage. I stopped bothering when I caught my drivers just pitting in every lap during a race (endurance).
If set to off they will keep going until either fuel or tyres run out. If the tyres wear out bob will automatically pit (have never run out of juice so I dont know what happens there). Its almost always tyres that will run out first unless you are playing some sort of fuel strategy. I have only found a use for this at nurburgring, you dont want your driver starting a new lap with less then 10% strength left or he will spend the majority of the lap with no strength and lap times will suffer as a consequence.
