not a mechanic, but looking through my manuals
for normal ops, the EIU selector remains on auto, if you don't have auto then your corresponding Left or Right
only 1 of the 3 EIU supplies data to all the EICAS displays
Left EIU, supplies, Left PFD, ND, and the 2 EICAS displays, Centre is on standby, Right supplies Right PFD, ND
Once the Left EIU fails, it will auto switch to the Centre, and if that fails, switches to the Right.
Auto switching is inhibited during a standby power situation
hope that clears the transfer logic