It's an emergency propeller, when the main engines lose power.
It's also the reason I never sit in the emergency exit row. The reason they give you extra legroom is so passengers can use the pedals that pop up from the floor when the masks are dropped. When they ask if you're willing and able to help on an emergency, this is actually the most common thing you'll have to help out with.
Everytime I've been in a crashing airplane, I just lean back a few degrees and laugh at those poor suckers who traded for a couple inches of extra legroom and now have the responsibility of saving the rest of us.
I can't believe everybody's just ad-liiiiibing!