Any bag you check will get a special tag that denotes it as a standby bag. The ramp will see the tag, and hold it aside. Close to departure time the gate will communicate to the ramp which, if any, standbys cleared and their appropriate bags will be loaded on the plane.

On DL, All nonrevs traveling domestic get 2 checked bags for free. A few international routes, like Lagos etc., Are limited to a single checked bag. Someone from CO would have to fill us in about CO's nonrev free baggage allowance.