And she notices if the arm holds up to being pressed down, or if it gives in and becomes weak. There is a certain knack to sensing it, but once one has got it, it is a very finite sign. One would pick a muscle that tests strong when we don't put attention on anything in particular. Then we will know that it means something when it tests weak when we bring up particular subjects. There are much more elaborate systems of using and interpreting muscle testing.
Different muscles seem to correspond to different organs along the lines of the meridians.
If you want to study and use that, that could be useful, but it is beyond the scope of what we get into here. We simply find a muscle that is usually strong and we agree with the person that it will be our test. This agreement becomes to governing factor. Whether a Yes or a No answer is most desirable depends on what we ask for, but most often we will ask in such a way that a strong response is good. If it is weak, we could then work on the issue of feeling good about oneself, with our usual processing techniques.