Hi alltorque,
Instead of thinking of a propeller in fluid, think of an airplane wing. What keeps an airplane gliding forward when the engine stops? It is quite apparent that an airplane maintains a positive AOA in a glide, yet it doesn't slow down then start going backward.
Remember that what causes lift is a pressure difference - think of it as a string pulling at 90 degrees to the relative airflow. With a gliding wing (or driving section of helicopter rotor), that pressure difference ends up pulling both up and forward. The change in the direction of relative airflow is very dramatic when you go from powered flight to autorotation, which can make the visualization somewhat more difficult.