Its nothing complex, looking through the translucent blue plastic case, it looks to be a simple VOM/Display Driver chip and a 4x 7-seg LED display block. Im guessing that the current monitoring is done after the supply for the LEDs is taken, as with no USB load it does read 0A.
I think its name of 'Charger Doctor' is supposed to indicate its intended use of checking when a USB port is used for charging a device. I have it now connected charging my Kindle
80-90mA seems to be what the Kindle is taking, and with this load the USB port is running at 5.07v. I did try earlier with one of the RTL SDR dongles, which took about 110mA, and the voltage dropped to about 4.9v
Yes, fairly frivolous. Yes, reasonably useful occasionally. Worth the £1.99 from the far east, free shipping? Absolutely.