I found an interesting Endwrench article from Spring 2002 at
http://endwrench.com/images/pdfs/LegacyandImprezaInfo.pdf. It describes how the ECU will cut fuel to only injector #1 if it believes it can't control idle. And the article says that some ECUs will also do this if the ECU senses a loss of power on the backup power pin.
I don't know whether this applies to turbo ECUs. If it does, then the logic involved might be modified to create a rolling fuel cut among the cylinders to support anti-lag or launch control.
