David Hawkins was well known for his research on understanding human consciousness and he is the author of books like Power Vs Force, Letting Go etc. In Power Vs Force, he talks about the scale of conscsiousness and the different emotions that are present at different levels.
These emotions are callibrating at different level of consciousness. So, the thing is that the muscle testing technique can be used to find out if something is true or not, or if something is good or bad(ie. if it lies above the level of 200 or below it).
Muscle Testing Technique
In this experiment a person can stretch out his arm and hold it firmly. Then the other person will press down the arm with 2 fingers. Before this happens the person stretching out his arm should hold in mind a thought, a person, or an idea. When the person does this, the other person can press down the outstretched hand with his 2 fingers. If the thought is true(ie it calibrates 200 or above) then the hand remains stiff and does not go down on applying force and the thought can be concluded as true, because a true idea or thought helps the hand remain strong.
Although if the idea is false, then the idea causes the person having his hand outstretched go weak and the hand will come down.
So, this is how you can use this technique, it is also called kinesiology.