I wanted to install an Intel 7260-N WiFi PCIe card into my T420. For this to work, I had to flash a modified BIOS that removes the hardware whitelist. Unfortunately, I had just upgraded to BIOS 1.49, which ostensibly cannot be downgraded.
The solution was to manually call winflash using the sv and sd flags:
winflash.exe /sv /sd 83ET76WW\$01C8000.FL1
The above command assumes that you're in the bios_mod folder, which is contained in the modified BIOS archive on ThinkWiki.