Last time I looked into this, some models came with UEFI or BIOS booting built into the board firmware and just needed a flag set in developer mode on Chrome OS

Last time I looked into this, some models came with UEFI or BIOS booting built into the board firmware and just needed a flag set in developer mode on Chrome OS, but on other models this code was either absent or too buggy to be usable. On those models, you needed to flash an updated/modded version of the firmware, which usually required removing a specific screw that doubles as a jumper to enable/disable writing the chip.