I see the term JTAG used for reviving dead Android devices that otherwise don't respond to anything and aren't seen when USB connected. It seems that it's something done from the physical hardware side of the device (as opposed to some pure software based solution) and I've heard the term used elsewhere (in gaming consoles). I don't find much else about it, and I'd like to know specifically how it relates to Androids and similar devices. It seems to be a term in electronics or electrical engineering, both of which I am unfamiliar with. I'm wondering what it is and why it can be used on hard bricked devices.
JTAG has zero involvement with Android!
You may find a JTAG being visible either under the battery or near the SIM/SDCard holder, its a circuitary pin-layout hidden, one may have to dismantle the back cover, usually 6 gold pins in a cluster close together.
It is that, using a special hardware box that has a vast-array of cables with designated pins (depending on hardware manufacturer), along with specialized software which would be specifically made for that particular handset (there are some generic catch-all kind of boxes that can be loaded with specific micro-code instructions for that respective manufactured board - the part is that can be tricky, is getting the identical micro-code for that circuit board, load the wrong set of codes then there's a risk of permanent damage!).
The micro-code instructions are emitted by the hardware box attached to the cable, clipped in position on top of the JTAG pin-outs, which sends a signal to some part of the chip to revive it, and then from there, a appropriate firmware can get flashed into place using this method.
This is a highly specialized topic where a background in electrical engineering would be helpful, to know which pins are either ground (GRND), transmit (TX), receive (RX) and also power (PWR). The power, is the important one, as if the battery is dead, power can still be activated into the circuitary.
Furthermore, those hardware boxes, which can be found on the internet are very expensive (due to differing manufacturers with different types of cables, and positioning of JTAG pinouts...)