In my company there is a procedure for CCM to shut off the engines in case of incapacitation during evac. Don't have the procedure here but from the way the instructors tell it in the safety recurrent they just teach them what the engine levers are and that you have to put it in off to put the engines off
. On 737 just look for the throttles and put the switches in front to shutoff. Logic behind it is indeed that a running engine can kill during evac, flaps are less important (no fire risk, suction, heat,etc).
Sounds very nice in theory but I'm not sure in real life how it will happen (stress, panic,....)