A feature introduced by Nintendo with the Game Boy Advance, this allows two or more players to connect via the link cable to play a game. Only one player needs to have the Game Pak inserted; the others download a portion of it. I don't know the exact baud rate of the cable, but it took about 20 seconds to transfer one track and car to another Game Boy Advance with F-Zero: Maximum Velocity. Games are not saved in memory afer power-down, as the Game Boy has no built-in storage. Furthermore, due to the size of the Game Boy Advance's Boot ROM, the downloaded image can be no larger than 256 kB (or 2 megabits).
A very nifty feature of the GBA, since it vastly increases the number of games that you and your buddies can play. If you are very cheap and know a Game Boy Advance owner, just call him/her up whenever you want to play. Link cables are cheaper than games, you know. :)