I have solved the mystery. The switch is wired so that the Bridge pup act as a master On-Off. If the bridge volume is moved away from zero, then the neck pup becomes active.
I am stumped! I just acquired a 25 year old Guild X-150D. It is great but I now have a problem. It has 2 humbuckers, 2 volume and 2 tone pots. When I select either pickup it works fine. When I select BOTH pickups (switch in centre position) only the bridge pickup plays. I have pulled out the...