JTAG is a Protocol !

Simply in other Protocols such as USB is  protocol UART is protocol for transferring DATA or communicating with devices .

JTAG Directly communicate with the CPU and with the help of CPU's JTAG Protocol you can access CPU's peripherals and memory/NAND/eMMC one of this peripherals .

In order to relate directly answer to your question for the reviving part .
Even Though your device is not able to boot-up from the internal memory/NAND/eMMC JTAG access still available through JTAG Protocol .

Accessing JTAG and re-writing Boot partitions to your device internal Memory/NAND/eMMC will revive your phone , phone will be able to boot-up and will be revived .

Some Commercial JTAGs : ort-jtag.com


Some JTAG Pinouts : 
[![JTAG Pinout][1]][1]


  [1]: https://i.sstatic.net/c9tA0.jpg