Hey there, I'm DanTheMan who made that video, and hopefully I can add a bit more info. Yeah, Youtube only allowed 480p at the time, and my digital camera had very limited video capabilities so I doubt re-uploading it would make it any better.
Anyway, the main link was actually already supplied by FAST6191, though in the slightly wrong spot on the page. This is where you want to look:
https://web.archive.org/web/20150810222526/http://reinerziegler.de/GBA/gba.htm#cart reader and
https://web.archive.org/web/20150302032339/http://www.teamknox.com/ula/ula.html
The software they provide lets you use the GBA as a USB joypad with a homemade USB cable built using the "ULA" specification. I've never been a big hardware tinkerer myself, so I never made one of those cables.
I did however happen to buy a Flash2Advance Ultra cart and it came with a USB linker cable. It appears that the commercial Flash2Advance USB cables were actually slightly modified ULA designs. Thus, the people behind the controller software then created a special file to make it work with the F2A cable rather than a homemade one.
That's how my video was made. I replaced/renamed the original "gba_boot.bix" with "gba_boot_f2a_lmt.bix" which made it work with my commercial cable, and that was that.
Note: it only worked AFTER I had installed the drivers to get the F2AU cable working in the first place, which to my knowledge only works up to Windows XP. The drivers will not install correctly on Vista (lol) or newer. It might work via a virtual machine or something, but I never tried it myself.