This technique is basically 'applied kinesiology':
Applied Kinesiology: Phony Muscle-Testing for "Allergies" and "Nutrient Deficiencies"
Quote from link above:
"The Bottom Line
The concepts of applied kinesiology do not conform to scientific facts about the causes or treatment of disease. Controlled studies have found no difference between the results with
test substances and with placebos. Differences from one
test to another may be due to suggestibility, distraction, variations in the amount of force or leverage involved, and/or muscle fatigue. If you encounter a practitioner who relies on AK muscle-testing for diagnosis, head for the nearest exit. Meanwhile, it would help if science-based kinesiologists would stop referring to what they do as applied kinesiology."