The blowing was to use the blown air to create the trailing edge if the airfoil so you could have a rotor where the clockwise edge was trailing when the rotor was spinning and then when you stop the rotor to fly the blades as wings half the blades blow out if the counter-clockwise newly trailing edge. Without this, you'd have half the rotor ballades trying to fly backwards when you convert them to fixed wings. It's a crazy idea that blowing through a slot on the lower aft edge of a blunt shape creates a trailing edge streamlined effective shape.