Multi-player PSP games with one UMD disc?

Thanks to Vap0ris for this interesting hack for adhoc gaming on the PSP.

To play any multiplayer game on a Sony PSP with only one copy of a game, do the following:
Place the game into the 1st PSP. Start the game. Start a Multiplayer Ad-Hoc game. (Set the time to unlimited because moving the game between the units takes a while.) Enter the game.
Once you are in the game, remove the game from the unit. A message will appear asking whether you want to exit the game, say NO! The game should resume.
Now take the game and place it into the 2nd PSP. Start the game. Now go to Ad-Hoc games and find the server that the other PSP has already started. Enter the game.
And now you are playing each other on two PSPs with only one copy of the game. And if you want to, you can do this with up to as many units as you wish. As long as the game supports that many players.
Enjoy!