This is to my knowledge not AK but some form of specialized kinesiology method you are describing. In AK one have to test the substance in the mouth.
I have studied both specialized kinesiology and AK for about 7 years and I would not trust or recomend testing a substance just by focusing on it.
Furthermore, a muscle that is normotonic but goes weak from a substance challenge, indicate that the body doesn´t want more of that substance at the moment. Weakening in this case is a sign of stress.
If a normotonic or weak muscle goes hypertonic, it´s indicating that the substance is bad for the person and that the body has developed an active defence of it. This is a sign of stress over a longer period of time.
If you test a substance that weakens and then ask the question "Is this bad for you/me?" The muscle should get strong indicating a "Yes".