Ummm...do they not all have computers hooked up to the scanner that they swipe your passport through? Perhaps, email, IM or some internal program is used to broadcast out the changes? No different than any other company needing to get time sensitive information out.

I'm sure they can also communicate with each other in case one notices someone/something suspicious in another line. They often look up and scan other lines where the LOS of other agents might be blocked.