The new BIOS in the SteamOS 3.5.1 preview update not only makes undervolting easier, but also safer.

Quick summary is the preview update channel now runs on 3.5.1 and gets bios firmware update 118.

Bios 118 includes undervolting settings in the bios by default (previously you had to run a command in desktop mode to enable them).

More importantly, doing a CMOS reset now resets the undervoltage values. So if you undervolt too far and your system won’t boot, it’s now possible to reset it without additional hardware. To do a CMOS reset you need to hold the volume down, QAM button, and power button with the device off.