Multivax.ca

The hack is a modified firmware of the Xbox 360 Hitachi-LG GDR-3120L DVD-ROM drive (the security in the Toshiba/Samsung TS-H943 is said to be similar, so it's probably also possible with this drive ... but it does require it's own hacked firmware of course).
As you (should) know, all Xbox 360 executables (XEX files) are signed by Microsoft (with a private key only MS has). This means that if you try to change anything to the XEX file, the signature will be wrong and the file will not boot.
Now ... to protect from booting an exact copy of a game from a DVD-R or other recordable media, microsoft gave each XEX file a 'mediaflag'. This mediaflag tells the Xbox 360 from which media (cd-r, dvd-r, dvd+r, dvd-rw, hdd, dvdxbox, dvdxbox360, ...) the XEX is allowed to boot. Changing this mediaflag in the XEX header is not an option as it'll break the signature of the file (see above), so ... what's done in this firmware hack is 'break' the detection of the disc.