On the outside, mum poker is the classic poker face, extended to the entire body, and maintained through sixth street.  On the inside, mum poker is no complaining, no blaming, no regretting.  Mum poker is stillness.  Mum poker is readiness.  If you wanted to go all the way with it, you could think of mum poker as being like absolute zero, the cessation of motion.  It is knowable in theory, and forever approachable, yet unattainable.