Dual-slot mode (Japanese: ダブルスロット Double-Slot), often called the Dongle method, is one of the many ways that players of Pokémon Diamond, Pearl, and Platinum can obtain Pokémon that are outside of the Sinnoh Pokédex, especially Pokémon that are exclusive to other regions, by using the GBA games of the series as dongles for the DS games. However, due to the removal of the GBA slot, dual-slot mode is not available if using a Nintendo DSi or a Nintendo 3DS.
The majority of Pokémon that can be obtained in this way are in some way version-exclusive Pokémon to one of the Generation III games. With one of the five Game Boy Advance games inserted into the GBA slot of a Nintendo DS while playing either Diamond or Pearl, players will be able to find Pokémon not otherwise available in Generation IV in certain areas of Sinnoh, but only after they have unlocked the ability to use Pal Park as well. One does not need to have the game in their GBA slot the whole time; one just needs to have it in at the main screen, at which time one can simply take the cartridge out, as long as one doesn't turn the power off before catching the Pokémon.
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Source: Experience & http://bulbapedia.bulbagarden.net/wiki/Dual-slot_mode