Remember always wing chun is a principle and concept based system.
Therefore there are numerous possible applications with-in the different movements, positions, and postures.
The dummy arms can be a right or left or both can represent one arm. The arms could be from wrist to elbow or elbow to shoulder. When using the both arms as one, one arm could be wrist to elbow the other could be elbow to shoulder. I also am of the opinion that the more you see something within the forms the more important it is and when it is performed multiple times in sequence it is even more important in that there are multiple possible uses. Don't compartmentalize the actions as specific techniques or applications. Stay open to multiple options. The rolling action of the arms could be as Mook stated, could be an arm break, could be just moving the arm off line.