Hi Dave,
It is helpful to look at 'everything' as energy. Regarding stress, or stressful thinking, 'creating' a blockage for instance... Yes, that could be the case. I would first say though, that if I was looking at someone holding stress energetically , the energy of stress itself is an extra energy that you do not need. You can sort of see that if you picture this, someone with, or without stress. So a blockage exists having stress itself, let alone what might further manifest from that.
Secondarily, holding this stress, over time, has an effect on your physical body and spirit, and futher blockages could develop from that. Say, an ulcer , or high blood pressure , or additional negative emotional reactions to increasing ill health, and then inturn, more blockages.

In your SFQ practice, one of the first 'tools' you are given to utilize is a 'focus'. Say you are stressed out. If so, you probably spend much of your time thinking about what you are stressing about. As soon as you enter your qigong exercise session, the focus you employ, is other than stressful thought. You stop that pattern of stressful thinking, as you focus. Although this can be somewhat a mechanical shift, it works. As long as you focus, you are not having stressful thoughts. Because of the focus, visualizations, alignments, within the exercise, you also self heal as you are in this state. So you begin to release this extra energy. There are of course, all shades of gray regarding how well you hold the focus, etc... Over time, through your practice, meditation, .... you gain wisdom, or understanding, further insights,,,, that further allow you to effect your thinking post session. More of an embodied understanding that even more effectively eliminates stress in your life. This is connected to the idea that qigong is a lifestyle. More self awareness develops in between actual exercise sessions. You might be sitting in your car, waiting for a stoplight to change, and find that you are experiencing a moment of self awareness, choosing to reverse breathe or something, and in the present moment not having a stessful thought, or, choosing not to and breathing instead, just being present.

A different scenerio: Let's say your friend has a tumor, and you want to apply this same theory of taking away the idea. Relative to this, you are taught to use signals or messages. So, as opposed to "I have cancer.", or "He or she has cancer.", you begin to 'change' the message to "No more cancer. , or "completely open.", etc... You don't use the word cancer. You change that, begin to neutralize it, by calling it a blockage. Just a blockage, some extra energy you don't need.
So, you take away that idea of yourself, or someone else, having cancer, and thinking that. This is a very important key to the effectiveness of SFQ.
After putting in that signal or message, then, the work is to support that, by not reverting to the thought of "I or they have cancer". You are taught to see the side effects or feel them or define them as "healing" sensations, or seeing whatever comes, as part of healing, not, having cancer and "it" coming back. So you don't want to look for what you just cleared, after a session. You don't go there. As you hold this level of clearity. Again, there are all shades of gray regarding effort and success. Everything improves with time and practice. The life style aspect is important. If you look at just doing the practice as an exercise session, period, you still are doing well for yourself, but it is difficult to progress, if say you are stressed 12 hours a day, and do qigong for 30 minutes. It takes very little, to spread the wealth, like that moment in your car. Qigong, doesn't have to just mean a fixed time and 'session'. Self awareness, mindfulness, can be experienced at any moment, and it is a part of this. Any time you have the thought come up, "I have cancer.", there is an opportunity to release and transform. If the thought completes, you could immediately let it go, afterwards. Like,,,," That thought was not good for me, I release it and send it back to the universe..." So , taking an action to change having the thought come up. Or, you can stop a thought midstream. If the thought doesn't complete, it doesn't have the charge to it that supports manifesting. It is a very selfempowering experience, to have a thought forming, become self aware as it happens, and stop it short of completing. It is even possible to see a thought coming , and choose not to have it, coming back to the present. Most of the negative thinking that goes on, is not relative to the present moment.
Aside for the message or signal, you have the nuts and bolts of universal qi to work with. You can literally effect blocked qi with sword fingers and so on. So you change the message, and, literally remove the energy blockage. At that point, you 'are' completely clear. In the case of a tumor, as physical manifestation of something, when you clear it, you don't see it as 'tumor' anymore, just 'healing'. The physical part "can" take awhile to break down. It is easy clear the blockage. Then you hold that clearity of mind, and continue to keep your energy clear through exercise. One day you will find the tumors gone. This timing varies from person to person. For some it is immediate, for some, a longer journey.
Master Lin gave an example of what happens when we die, regarding this yin and yang concept. He said the spirit is yin, and the body yang. The spirit leaves the body right away, and the body, the physical part, takes time to break down.
love,
gallen