What was the dual-slot mode?

The original Nintendo DS and DS Lite carry two cartridge slots, one for DS games on top and one for Game Boy Advance games underneath. Generation IV's designers treated that lower slot as an expansion port, and the feature is called dual-slot mode. It does two jobs in Diamond, Pearl, Platinum, HeartGold and SoulSilver, and both die on any system without the slot, which means the Nintendo DSi and the 3DS family cannot do either.

The famous job is migration: a Generation III game below lets Pal Park pull six Pokémon at a time up into the DS game, permanently. The quieter job is nicknamed the dongle method, because the Advance game works like a hardware key. Leave one in the slot when you start the game, earn the National Pokédex, and the wild population of Sinnoh changes around it.

Each Advance game unlocks its own strays, usually species that were exclusive to it. FireRed in the slot puts Elekid on Route 205 and at the Valley Windworks and Growlithe on Routes 201 and 202. LeafGreen answers with Magby on Route 227 and around Stark Mountain and Vulpix on Routes 209 and 214. Any Generation III cartridge at all adds Haunter to Eterna Forest. Unlike migration, the dongle method ignores language entirely; any region's cartridge works.

It is a very 2006 idea of downloadable content: content that shipped on a cartridge you already owned, unlocked by owning it. When the slot vanished from Nintendo's hardware, the strays simply became unreachable, and the cartridges that summon them turned into keys to a room that newer systems no longer have.