How it work in fluff? Anybody see it (or its analog) in real life?

Fire selectors are basically just several clips attached to the same weapon. The space marine toggles a switch, and a different magazine becomes the primary sources of ammo.

Personally, I imagine them to look like the box magazines these Sternguard have modeled on them:

http://www.games-workshop.com/gws/catalog/productDetail.jsp?prodId=prod1620032&rootCatGameStyle=

I can see probibly a box magazine instead of a regular sickle magazine and inside the box there are 2 or 3 seperated sections each loaded with a different kind of round. The shot selector would let the weapon decide which section of the magazine it takes ammunition from.