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The back up battery switch is not used in the normal sense. The nomenclature is off of what we normally think, possibly a result of the German translation?..
In this case it is a switch that is used to keep the engine running long enough to get you on the ground in one piece in the rare event of a double Alternator/Regulator failure. There has been a lot of discussion around this on the Rotax forum which I highly recommend joining if you are a Rotax owner/flyer. Here is a link to one of the threads on this issue: https://www.rotax-owner.com/en/912is-technical-questions/5822-backu...
Thanks joe, that explains the confusion. Probably the result of a translator who is not a technical person!