Normally to unlock the bootloader of a Xiaomi phone you would need to first register your phone with Xiaomi then wait 2-3 weeks for the unlock code. Developer "wolkenjaeger" ran into an issue with his device and did not have time to wait for the code so he figured out how to unlocke the bootloader on his own.
The step by step instructions seem pretty straight forward. In order to proceed you will need adb and fastboot installed on your PC. You will also need a specific Mi4c image. You will then reboot into fastboot mode, and then follow the rest of the steps in the complete guide below .
via XDA