in a variable pitch propeller a point of the blade is fixed to other points on the blade, so its shape is always constant. The blades are fixed (usually bolted) to a central hub. The blades move relative to the hub, which causes the face of the blade to be at different angles to the hub centre line. When the pitch face angle is at 90 degrees to the hub, it is in a neutral position and will not generate a force (either forward or astern). The further away from this neutral position will cause a greater force. Moving a propellers blades in this manner is like having propellers of different pitches fitted, but without having to change the propeller.