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JTAG and Android are two separate things.

You may find JTAG pins visible either under your device's battery or near its SIM / MicroSD card holder. The pins are hidden. You may have to dismantle your phone's back cover. The JTAG pins are usually six gold pins clustered close to each other, as shown in this photo:

You use a special hardware JTAG box which comes with a vast array of cables (different cables for different types of devices), plus specialized software specifically made for your device model. (There exist generic catch-all JTAG boxes that can be loaded with specific microcode instructions for your particular manufactured board. The tricky part is getting the exact microcode for your device's circuit board. If you use the wrong microcode, it can cause permanent damage.)

The microcode instructions are emitted by the JTAG box attached to the cable, clipped in position on top of the JTAG pins. The box sends a signal to your device to revive it. Now an appropriate firmware image can be flashed to your device.

This is a highly specialized topic. A background in electrical engineering can be handy, to know which pins are used for ground (GND), transmit (TX), receive (RX) and power (PWR). The power is the important one: if the battery is dead, power can still be fed into your device.

You can buy JTAG boxes online, but they tend to cost a couple hundred US dollars or so. It's because of differing manufacturers with different types of cables, and different arrangements of on-device JTAG pins.

JTAG and Android are two separate things.

You may find JTAG pins visible either under your device's battery or near its SIM / MicroSD card holder. The pins are hidden. You may have to dismantle your phone's back cover. The JTAG pins are usually six gold pins clustered close to each other, as shown in this photo:

^{Click to enlarge}

You use a special hardware JTAG box which comes with a vast array of cables (different cables for different types of devices), plus specialized software specifically made for your device model. (There exist generic catch-all JTAG boxes that can be loaded with specific microcode instructions for your particular manufactured board. The tricky part is getting the exact microcode for your device's circuit board. If you use the wrong microcode, it can cause permanent damage.)

The microcode instructions are emitted by the JTAG box attached to the cable, clipped in position on top of the JTAG pins. The box sends a signal to your device to revive it. Now an appropriate firmware image can be flashed to your device.

This is a highly specialized topic. A background in electrical engineering can be handy, to know which pins are used for ground (GND), transmit (TX), receive (RX) and power (PWR). The power is the important one: if the battery is dead, power can still be fed into your device.

You can buy JTAG boxes online, but they tend to cost a couple hundred US dollars or so. It's because of differing manufacturers with different types of cables, and different arrangements of on-device JTAG pins.

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.

Back of Zte Blade http://img843.imageshack.us/img843/2452/dsc0178.png

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...)

JTAG and Android are two separate things.

You may find JTAG pins visible either under your device's battery or near its SIM  / MicroSD card holder. The pins are hidden. You may have to dismantle your phone's back cover. The JTAG pins are usually six gold pins clustered close to each other, as shown in this photo:

You use a special hardware JTAG box which comes with a vast array of cables (different cables for different types of devices), plus specialized software specifically made for your device model. (There exist generic catch-all JTAG boxes that can be loaded with specific microcode instructions for your particular manufactured board. The tricky part is getting the exact microcode for your device's circuit board. If you use the wrong microcode, it can cause permanent damage.)

The microcode instructions are emitted by the JTAG box attached to the cable, clipped in position on top of the JTAG pins. The box sends a signal to your device to revive it. Now an appropriate firmware image can be flashed to your device.

This is a highly specialized topic. A background in electrical engineering can be handy, to know which pins are used for ground (GND), transmit (TX), receive (RX) and power (PWR). The power is the important one: if the battery is dead, power can still be fed into your device.

You can buy JTAG boxes online, but they tend to cost a couple hundred US dollars or so. It's because of differing manufacturers with different types of cables, and different arrangements of on-device JTAG pins.

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.

Back of Zte Blade http://img843.imageshack.us/img843/2452/dsc0178.png

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). 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...)

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.

Back of Zte Blade http://img843.imageshack.us/img843/2452/dsc0178.png

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...)