There was a group of bochurim who made it out of the Soviet Union who me with R' Moshe (I believe in the '50s) and after hearing about the mesiras nefesh they had to keep and teach Torah under such conditions, he asked how they did it. One or two gave ideological or philosophical explanations which he did not accept until R' (? I forget the first name) Notik said in a questioning tone, "א ברירה האבן מיר געהאט?"
Reb Moshe grew up in Russia and knew what the hardships were, and was amazed that Yidden stayed strong for so long. He accepted and appreciated the other answers (chassidus, farbrengens, the hope to see the Rebbe etc) but was blown away by the simple answer of Reb Yankel Notik (later lived in nachlas har chabad)
A chossid was imprisoned in Siberia refusing to eat non kosher food. There was another fine Jew who was berating him, telling him that halachically he is allowed to eat it. Years later he said, "ער זאגט אז איך מעג עסן, אבער ער פארשטייט ניט אז איך קען ניט עסן"
From chabadinfo:
Two years ago, Rabbi Mangel made a trip to Poland together with the Boyaner Rebbe and hundreds of his followers (see sidebar). One of the places that they visited was a place called Plaszow, not far from Cracow, where thousands of Jews were slaughtered and buried in a mass grave.
Rabbi Mangel stood there with the Boyaner Rebbe alongside him, surrounded by the hundreds of Chassidim. They asked him to share a few words, and with his gifted oratory he did so and shared it with us:
I will speak about the kiddush Hashem and mesirus nefesh of Jews in Auschwitz. There are people who think that whoever came to Auschwitz lost his faith, so I will tell you of a few cases that I witnessed or heard from people firsthand.
When I was in Auschwitz, as mentioned, I slept in the first position on the bunk. Behind me slept my father, and behind him was the rav of Pressburg, a Jew named Schreiber, a descendant of the Chasam Sofer. He was a very refined Jew and a tremendous Torah scholar.
Each day they would give us a half a loaf of bread, which was already mostly moldy, and a little bit of “soup.” This was nothing more than hot water with a few kernels of barley, and thin strips of horse meat. Rabbi Schreiber refused to partake from this.
My father said to him, “Pressburger rav, it is pikuach nefesh (danger to life).” His answer was, “I can’t do it.” My father did not let up and said, “This overrides the entire Torah.” But he kept insisting, “I can’t do it.” My father then went on, “It does not just say ‘overrides,’ it also says ‘permits,’ it is literally permissible.” He still would not budge, “I know all that, but what can I do, I am incapable.” So, he only ate the moldy bread.
After two or three weeks, he passed away.
I said to the Boyaner Rebbe, “He was a righteous Jew. He did not say, ‘It is forbidden to me,’ he said, ‘I can’t do it.’” I gave an analogy: If there was a situation of danger to life, could you eat rocks? Obviously not. For him it was literally impossible. I said that this was akin to the level of Moshe Rabeinu. When Hashem said to Moshe, “go down and warn the nation” not to come to close to the mountain, Moshe answered, “The nation cannot ascend.” What does that mean? They cannot? Obviously, they were capable of doing so… However, on the level of Moshe Rabeinu, when something is forbidden – it is impossible!
