Pit Strategy determines at what level of Strength he will pit in, where he will pit with about 10% left when you set it to 10, 20% at 20, and so on. Alternatively, whether you've set Pit Strategy or not, he will pit when his Tires or Fuel are almost out.
With the X1, it burns through fuel so quickly that your backup drivers won't have time to recover their Strength before getting back in, so having any Pit Strategy set will cause the drivers to pit sooner and sooner until they're pitting every lap. It's better to just leave that off and let them suffer until they need Fuel. On cars that don't stick quite as well as the X1 such as LeMans Prototypes, 10% would be a safe bet, as the drivers will start having trouble keeping the car on the tract when they get too low.