[rodgerd]

> How can they know/figure it out?

Sony's answer for Xperia phones used to be that you'd unlock the bootloader with a key you got from them by entering the serial number/IMEI. As part of that, they warn that using custom ROMs that damage the phone - by overclocking, for example - would void the warranty.

It's a reasonable compromise, in my opinion.

[selimthegrim]

Do they still do this?

[brokenmachine]

Lenovo does