Many bricked PDAs cannot be booted with full ADB support while FastBoot mode is accessible. But if bootloader is locked, there is no way to flash a partition or boot an image.
Some firmwares support rebooting to EDL in Android Bootloader aboot module (the "reboot-edl" command) but fastboot utility doesn't recognize "reboot-edl" in the command line and refuses to sent the command to the device.
Since FastBoot command protocol is simple and text-based, it is enough to send a packet containing "reboot-edl" text over USB. The simplest way to do that is to patch Windows fastboot.exe file, replacing "reboot-bootloader" text with "reboot-edl". As "reboot-bootloader" string is found in the command line, fastboot.exe sends the same constant string to the device. So if "reboot-bootloader" is replaced with "reboot-edl", you can specify "reboot-edl" in the command line and the same text will be sent to the device.
In the attached example, the mentioned string is located at offset 2685c.
I occasionally locked bootloader in my unlocked/rooted Xiaomi Redmi Note 3 Pro (Snapdragon) so it could boot to FastBoot or Diagnostic 900E modes. Tried to swich from 900E to 9008 with QPST but it reported unspecified error.
Analyzing aboot (emmc_appsboot.mbn), found that it supports "reboot-edl" so the only remaining thing was to find a way to send it to the bootloader.
With the attached fastboot.exe, successfully rebooted to EDL mode (9008) and re-flashed proper partition images.
Of course, patched version does not support "reboot-bootloader" command because it has been replaced by "reboot-edl".
There also is a detailed guide.