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 :
