If I view HTTP headers, I see many requests being made constantly during playback. What is all that for?

Each HDNeStream class will set up two concurrent HTTP sessions to the Edge server. The first, called the delivery session, is the one that delivers the actual video content. This stays connected as long as the video is being downloaded and played. The second session, called the control session, is used by the class to communicate with the server. This communication channel allows it to control the buffer length on the delivery session and also to switch the content that is playing on the delivery session to another bit rate. If you have the NetSession client installed on your machine, then you will see additional requests going out to that service to query it for the current CPU usage value, which is a metric that Flash does not provide but that the class can make use of in its switching decisions.