Hi,
I presume you mean the Engine Start Switches (not the levers!)
There is really very little chance of flame out during an emergency descent.. its simply an idle thrust, at Max speed, with speedbrakes out descent.
The start switches go to continuous in the event of entering icing conditions during the descent, and yes they also provide additional flameout protection but as I said, its not any more likely to happen during a descent as any other time