I'm not sure - I would say it could be the overtake boost or something, a feature that was incoorporated in the Turbo-era, but it doesn't appear to be linked to anything, and I don't think the overtake boost was a simple switch like that. It's not the right kind of button for something like that either. As LS said, it's more like a light switch
It doesn't look very technical does it?
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