Wii:How to Play DVD Movies (DVDX)

Today
The newer Wii media players, Wii:WiiMC and WiiMC-SS, can natively play DVD-Video (and files off data DVDs) as long as they're running in an unlocked AHBPROT environment (i.e. a recent version of the Homebrew Channel and an appropriate meta.xml).

Back then
The original DVD access solution for homebrew, created by Team Twiizers, was libdi in August 2008. At the time, it leveraged a helper title called DVDX (titleID: 00010008-DVDX, later 00010008-DISC to evade the first blacklist attempt) which would inherit the running homebrew; the trick was in DVDX's permission flags, which enable DVD-Video commands. Then it was just a matter of designed-for-purpose homebrew, mainly contemporary leading media players MPlayer TT and CE, and some emulators, to implement this interface.

It was later discovered another permission bit enables the coprocessor to directly pass through all sorts of devices to the CPU, the so-called unlocked AHBPROT state mentioned above; while poorly understood at the time, after being added to the HBC along with a mechanism to pass it to loaded software, it led to the deprecation of DVDX (and, eventually, of most scenarios requiring a patched IOS with the trucha bug!)