Bait Runner Reels?

Hi
Having been away from fishing equip in a while, I was wondering if someone
could explain (or lead me to a site) how these spinning reels are "bait
runners"? I see various makers, swtarting with Shimano selling these.
Do they permit free run of line by wsay of free spooling? If so, how do you
engage it when you want to set the hook?
Is there something about the placement of the drag controls?
Thanks VM