My Breadwinner Limited has one-Ovation billed it as making the guitar sound more "funky." It runs both pickups in an "out of phase" configuration that causes the guitar to sound a little more percussive and cutting to my ear.
It's the fabled suck switch. The printing on your control plate has probably worn off. It should say: Suck - on, off.
To be more technical... It puts the two pickups out of phase (suck on) or in phase (suck off). I believe it doesn't do anything at all if you have just the bridge or neck pickup slected. I've actually been able to use the suck on and get a usable tone with enough bass boost, but I'm not sure why you'd really ever want to use it.
Do not confuse this switch with the holy grail of all switches: the Don't Suck switch.