One way to picture this is to think of a propeller. As the propeller spins, it carves out a 3D space even though the blade is "2D" in a sense.
If we spin everything around segment BC, we get a 3D cone forming. The base of the cone is vertical and it has a radius of AC. The height of the cone is segment BC. It might help to rotate the paper 90 degrees clockwise so that BC is vertical.