Nope, this has nothing to do with the Game Cube game Mario Kart: Double Dash. This has to do with having files from two different dashboards on the Xbox hard drive. This is the basis of some newer hacks, including Easter Egg Exploit (virtually identical, but a bit less intrusive on normal system operations) and UDE2.

• Page 1 of the thread pointed to by Angerwound's pinned post linking to different exploits
• As I recall, my attempt to install this on someone's Xbox resulted in an Error 21 everytime the Xbox booted from the hard drive. Fortunately we could still boot to MechAssault and fix the problem.
  • As I also recall, I spent some time trying to find out what signature this uses, and maybe I even did not find the answer and just guessed (hence getting the Error 21).
  • The Xbox was a kernel 5713 Xbox. Further research has revealed to be that kernel 5713 is probably incompatible with this hack.
• Now that I've found some more information and have a little more experience with this, I think this is just the Easter Egg Exploit ("EEE") as described on this site's "EEE" page, except that the executable in the package gets called C:\xodash\xonlinedash.XBE
  1. Obtain devz3ro's package from "The Usual Places" at /XBOX/exploits/Font_Exploits/doubledash/doubledash.rar
  2. Do not follow the directions inside that package. (I am just not a big fan of deleting all files from the C: and then trying to copy them from a CD, especially since some Xbox drives cannot read some burned CDs as well.)
  3. I suggest first trying EEE, then if you want to enter this hack by selecting the "Xbox Live" tab on the main menu of Microsoft's official dashboard, to see if EEE works. If EEE functions properly and you'd rather have the "Xbox Live!" option on the main menu run unsigned code instead of Microsoft's "Xbox Live!" executable, continue the next steps.
  4. Ensure you have a kernel version less than 5713, because I do believe 5713 would fail.
  5. Back up and delete, or just rename C:\xodash\xonlinedash.xbe
  6. Either backup and delete files, or just rename files so that there are no C:\*.xip or C:\xtf files.
  7. Extract the contents of /C/ directory of doubledash.rar to the C:, including the C\xodash\xonlinedash.xbe which should be backed up from an earlier step.