Catholics are expected to utilize their rational nature in all matters. Health and medicine is usually a domain of science, where treatments are carefully tested to ensure their safety and effectiveness.
Faith can inform the limits of medicine, however, faith cannot be used to judge the effectiveness of a treatment, only whether the desired result is moral. For example, faith teaches us that life is sacred, and that thus terminating a pregnancy is immoral. Faith tells us the result, the death of an unwanted fetus, is immoral, whatever means are used to achieve that result.
With “applied kinesiology” as you describe it, you are attempting to use magnets to treat illness. While the goal is certainly moral, the method has no rational scientific basis. The method requires belief in a hidden force to achieve achieve healing.
This requires placing one’s faith in a supernatural force. We are taught to place our faith solely in God.
As God created Nature, we are free to science to learn how to use nature to cure illnesses. Yet, magnets have never been shown to cure anything! Using magnets to cure illness risks delaying effective treatment, potentially making a condition worse through inaction. They may even cause positive harm if someone has a pacemaker or other medical device that might be damaged by a magnetic field.
Thus, to use magnets to treat illness, we are ignoring scientific evidence, and actively placing patients at risk! This is not rational, and thus sinful. When we ignore our rational natures, and put our faith in random forces with no known origin, we harm ourselves spiritually by placing our faith in something other than God and his creation of nature.