The Chipmunk USB Tester is meant to be an easy way to see, through the USB port, if the computer is powered and if the CD321x is alive, rather than a device to 'test' the USB port itself, although you could use it for that too.
So whenever you are working on any computer or loose motherboard plug in the Chipmunk USB-C Tester into every USB-C port. This will only take two seconds of your time, but will give you valuable information about what is going on with the computer. The tester is narrow enough so that you can place two testers in two ports side by side.
A big and dangerous problem on the modern USB-C Macs is that the motherboard can fail in such a way that the USB-C port permanently supplies 12 or 20 volts (on VBUS) which immediately fries most external USB devices. The Chipmunk USB-C tester survives that, but more important, it will alert you to this dangerous voltage by blinking the red 'Power' LED.
One red LED: There is at least (some) power.
One red LED and the other red LED is blinking fast : There is power on VBUS but it is MORE than 5.4 volt!
Two red LEDs: The USB-C port is supplying 5 volts on the VBUS line..
Blinking green LED: The USB port is supplying data.
For ease of use the set of 3 LEDs is mounted on both sides of the board. No matter which way round the motherboard is on your desk you can always see the LEDs.
The VBUS LED (red) lights up whenever there is power supplied to the USB port. Any power above about 2 volts will light up the LED. This will show you at a glance if the computer is powered.
(If this LED blinks it means that the computer is pulsing the power on VBUS. There will almost certainly be a short in the computer somewhere.)
The 5volt LED (red) will only light up if the voltage is actually 5 volt. This will show you that at least the 5 volt rail is working fine. Below 4.75 volts it will be off, above 5.4 volts it will blink to warn you.
(If this LED blinks you should NEVER plug anything important into that port as a USB stick or hard disk, etc, will immediately get fried!)
The USB LED (green) will blink if there is any activity on the USB bus. If this LED blinks it implies that the CPU/PCH is running. A bit like the Rossmann 'Fan Spin' but better.