In that particular configuration with the neck tone 'up' (the series setting) it disconnects the link from the bridge pup to the bridge volume and reroutes it into the neck pup. So, with the pup selector in the bridge position the bridge pup isn't actually connected to it and therefore no output. However, with the selector in either the middle or neck position you should get both bridge and neck pups linked in series with the neck volume acting as a master volume etc.