Forgive me if i'm wrong, but AIUI, the Rotax employs electronic ignition modules. one assumes that these work by disconnecting the power-feed (switching off) Some transistorised (solid-state) stuff doesn't take kindly to shorting -out or open-circuiting..
When using ICO to stop, is the carburettor float-bowl still full of fuel?
If so,it's not much different, strangling the idle -fuel, to switching off the sparks.
Just as a mag-system will show a slight drop when each mag is grounded, both off will produce a sudden and distinctively quiet drop. A very brief transition through that switch-position is all that's required to confirm both mags are grounding-out correctly. under those circumstances,a backfire is unlikely..
Always treat a prop as potentially LIVE and you won't get a free lobotomy.
