What it does is it shows the screen saying you can change your gamertag to w/e.
Ex:
It says "Current Gamertag: Whatever"
"New gamertag: Pro"
It gives you the "Can't retrieve information from Xbox LIVE. Please try again. Status code: 80154000", so you can't accept the gamertag!
Tutorial:
1. Press the Xbox Guide Button
2. Scroll to the "Settings" tab.
3. Click Profile
4. Go to "Edit Profile", then "Gamertag"
5. Click Get Suggestions, then rapidly press the (A) button
6. It should bring you to a screen saying "You've chosen as your gamertag"
7. Then click "No, edit it"
8. Enter whatever gamertag you would like.
9. It should say "This gamertag is already in use. Try another gamertag or pick a suggestion", but ignore the error and press the (B) button.
10. It should bring you to a screen with the gamertag it said was unavailable, then hit the (B) button.
11. It should say "You've chosen "the gamertag you entered in step 8" as your gamertag"
12. It doesn't let you use the gamertag, but this can be used to prank friends, show off what you don't have, etc...