How does the 5th/6th slot work?

I have never really tried to bother with it, until now.

I don't really understand how this works after you activate it, can someone explain?

yea,first go into item shop and buy a 5th slot gun ak47 w/e,hit hotkey for 5th slot and you will see your gun in 5th slot.put it in your guys 5th slot.but when you shut game down and restart. you will have to load 5th slot when loading game,when you are picking a character(med,w/e)hit hotkey for 5th slot and there ya go