Chaza"l say that if a person is walking and he needs to say Kriyas Shema, he should "stand" and not continue walking when he says the first pasuk. The Beis Ahron of Karlin explains this to mean that one should stand firm like a beam of iron, meaning that when one accepts upon himself the yoke of heaven completely, he is able to stand up against any desire, even the greatest among the great. And, he continues, this is the meaning of true mesiras nefesh.