You know, this is an excellent question and I have thought along similar lines.
Under the "for what it's worth" dept, and also, "your mileage may vary", here is what works for me:
I think the butterfly imagery (which Master Lin has a whole meditation based on), is twofold in purpose: a) to substitute something more beautiful and vivid for smoke or air when the excess energy (which Master Lin explains is all every disease or pain is) visualized as leaving our body and b) the transformative symbol of the butterfly (i.e. the dis-ease which is just excess energy, turning from "larvae and pupae" stages into the butterfly and leaving the body.
Having said that, those of us with any kind of "low energy" problem might seem to have a dilemma (as per your question), "But what if I need MORE energy?"
I stay with the image of the butterfly as being the blockage leaving my body. And even a condition such as CMF or other low energy issues, can still be understood as being blockages in the flow of energy, not merely the lack of energy. So first, you want to clear the blockages and then bring in fresh energy.
So for fresh energy, I image colorful light coming into my body into every pore, opening, cell, molecule, atom.
And for something like a qi deficiency in the kidney or lung, I might send myself ( or someone else) colorful light
RIGHT INTO THAT AREA (after clearing it first). So for example, I might send using my sword fingers, bright blue light into my kidneys, or bright green light (ala Green Lantern for any fellow comic book fans out there
) my liver with my palm.
I feel like part of the imagery of the butterfly is for something to fly away from, not fly into (where it would be trapped).
However, light and color is very good (for me) to bring fresh energy into the body.
This is just one approach though, and I am still learning a lot, so I would be interested in other approaches.
I hope that helps you. Helps me!
