Good question Rockchuck and great answer Bluebird. Just to add some more practical advice....
We often keep several things going in our mind at the same time. The modern term is multi-tasking but in fact our minds have always been incredible at thinking about many things at the same time. In general people can think about 4 to 7 things conciously at the same time. Subconciously all kinds of things are going on.
By doing the active exercises you are getting your brainpower to start filling up those 4 to 7 concious thoughtways with good positive visualizations. Feel the energy in your hands, breath in good clean energy and exhale negative energy, do reverse breathing, feel the front and back channel meeting in your lower dan tien, see a bright light in your lower dan tien and so forth. As your relax, your mind seems to slow down and ultimately you will have only 1 or 2 thoughtways goings and if successful you will move into the quietness and then into the emptiness where you can keep your mind clear and have no concious thoughts. This is where you really want to be so you can maximize energy flow and healing.
In all honesty it is not easy in the begining to reach the emptiness or stay there for very long. Our babbling mind just doesn't want to stay quiet. You have to be a detached observer and just watch the thoughts keep bubbling up and then gently release them like baloons floating off into the sky and clear your mind again. Try not to move or shift your physical body as this seems to get the mental chatter going again. Be patient with yourself and know that it takes time to get to and stay in the emptiness. The more time you can spend there, then the more powerful your progress will be. In fact instead of thinking as we do in our slow physical world, in the emptiness you will be experiencing the world at the speed of light and great flashes of insight and experiences will be yours.
Agent B
