Does anyone know how outlane "Shields on" works in a Data East Star Trek?

There appears to be no setting for it. From what I have read so far a few people think it comes up when you shoot the right hole (scanner) and some think that's only in multi-ball mode. I have never seen it as a possibility even when repeatedly placing the ball into the scanner hole.

???

OK so I now see that when you enter multiball mode "Shields on" lights for like a few seconds. Then it goes off and the DMD & speech says "Shields Down".

Anyone know if that's it or there's more? Seems like there should be a timer for when you first start a ball like in other games...