You must make the extra wire bridge shown here:
You also need to solder the two standard write-enable points, but in my experience (just one of these) the xbox will not boot if both points are bridged. So, bridge the bottom one first and turn the xbox on. Then bridge the top one before you try to flash. (I soldered it while it was on) You can also add wires to the top point to connect after booting.
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