I had the necessity to take a look in more detail at the functionality of the VEX power expander today and though others may find the results interesting. The power expander, as I'm sure everyone knows, allows up to four external RC servos or motors to be powered from an additional battery rather than the main system battery. Inside the power expander a microchip PIC microcontroller provides all system functionality. The PIC not only gives status feedback by means of a tri-color LED (actually red and green leds that show yellow when they are both on), but also monitors the external input voltage, the voltage after the polyfuse and also the power coming from the cortex. The PIC is able to enable and disable power from the external battery to the motors so as to follow system power from the cortex.
External voltage detection has two thresholds, once the voltage drops below these levels the status led is set until the system is reset by power cycling.
Green LED, external voltage above 6V
Yellow LED, external voltage between 6V and 5.3V
Red LED, external voltage below 5.3V
The PIC is powered from a 5V linear regulator, an external voltage of 5.3V is probably right at the threshold for creating a stable 5V for the PIC.
Input PWM signals are routed directly to the output, all grounds are tied together.
Here is a simplified block diagram (none of the passive components are shown).
I make no claims for this information to be 100% accurate, the diagram is drawn from my own observations so use at your own risk.