Here is an Ohr Sameach version of this interesting story, thanks RebE
Rabbi Moshe Feinstein, zatzal, one of the foremost halachic authorities of our generation, was known to stand stock still during the silent prayer. He explained that, while living in Russia, he was once arrested for teaching Torah. One form of torture he experienced during his imprisonment was being forced to stand completely still facing a wall. The threat was that if he were to move he would be shot. It was on one of these occasions that Rabbi Feinstein was struck with the realization that if he could stand with such intense concentration for the sake of his captors, then he should afford at least the same respect when standing in front of Hashem.