To use it, download twrp-3.3.1-0-bullhead-4core.img and boot it with fastboot boot twrp-3.3.1-0-bullhead-4core.img from the stock bootloader.
Changed extracted/configs/recovery.img-cmdline to 4 Cores according to 4Core-Android-0-5X boot.img. The changes are recorded in cmdline.patch.
This way TWRP will work on bootlooping Nexus 5X devices, which are broken due to their big CPUs failing due to production mistakes. See the xda-developers thread under Resources below.
mktool is used for unpacking & repacking the originally release TWRP recovery image.
Project page for mktool: https://intechgeek.com/mktool/