The rule is US CBP Short Time Crew Turn Around procedures or the "90 Minute Rule." Unfortunately, I cannot find any publicly documentation for it.
The rule is that if you are a US crew on the ground at an international station for less than 90 minutes doing a flight US-international-US, you do not have to clear immigration. A delay that pushes the flight to spend 91 or more minutes on the ground requires the crew to clear immigration.