From WikiTemp, the GBAtemp wiki

The SuperKey for Nintendo's DS line of handhelds is the perfect solution for accessing unsigned Nintendo DS code from any GBA flash storage device. Simply plug in the SuperKey cartridge where you would put a regular DS game and make sure your SuperCard or other development kit is installed as well, and you're ready to go. You can run homebrew applications, movies, pictures, music, and games in DS mode, if its available in the software you're trying to run.

The SuperKey is compatible with almost all dev kits available on the market, including the M3, Neo Flash, EZ4 kits, and of course, the SuperCards.

A major advantage of using the SuperKey over the old SuperPass series, is the fact that the SuperKey requires no programming whatsoever. Simply plug in the SuperKey where you would put a normal DS game, and you're done. The SuperKey is a 3rd generation SuperPass, so it retains the same quality as a SuperPass would but requires no programming. Also, the SuperKey does not stick out, where as the older SuperPass series would stick out quite a bit.

Like the Passcard3, the SuperKey supports sleep mode, so as long as you have the SuperKey inserted in your DS, you can safely close the DS and not worry about your screens getting damaged.