I think they don't use engines to slow down. I have seen some parts of the wings (like the rear half of the wings..) along its length standing up right ( something like right angle to the direction of motion of plane) putting a lot of resistance to the air while planes need to slow down.

Using the engine to slow down is like using the reverse gear of your car to slow down, which no one does, which never works (because of possible breakdown of the engine) and should not be done to save the engine from damage.

[ I am not a mechanical or automobile engineer. But this is my knowledge ]