Just stick it into the DS and turn the DS on, truly Plug&Play device. I don't know the technical side to how it works, if thats what your asking though.
If you plan on getting one, it doesn't work with some flash carts, so I'd check compatibility beforehand.
Works with the all the M3 range (CF, SD, SD Slim, Mini SD) also works with the whole G6 Flash range. and works with the firmware-hacked GBA Movie Player. Probably works with other flash carts too, but thats all i know of.
It works flawlessly with all M3/G6 products, but according to WINSUNX it doesn't work with EZIV Lite, so it may not work with some of the others.