Spring loaded Stop-RUN-Start Switch. Default is RUN and in that position, it keeps Ignition and Fuel ON, along with all manner of "safeties." Might be checking for RPM, Oil Pressure or Level, AC and perhaps 12VDC voltage etc. etc.
Our old ONAN BFA didn't have a Starter Motor. The Armature incorporated a Start Winding along with appropriate Brushes. Elegant but risky and expensive. If that winding fails, gennie-no-crankee, and it's a $thousands Armature instead of a $50 starter from Amazon. Still, no Starter Drive/Pinion to worry about damaging against the Flywheel by hitting Start when it was already running. I used to hold the BFA in Start till its running steadied up. Reluctant to do that with our newer 4KY with Starter and Plastic Pinion Gear.
In Start, the genset gets Fuel, Ignition, Starter and not much more. Stays that way till released to RUN, when all the safeties kick in. So... hitting Start probably re-boots all the safeties and they might just find themselves and get producing.