A common misconception is the myth about having your chinstrap unbuckled. It is mainly an American thing that was spread in WW2. Fact is, if the concussion was strong enough to hurt your neck or face because your chinstrap was buckled, the force of the same concussion would more than likely kill you. Having your strap undone just meant you would spend a lot of time holding your helmet on while moving fast.
As the men are told to "Stand Down" most of them have their chin strap under their chins. Any experienced soldier would not do this because if a shell landed close the concussion would rip the helmet up and rip their faces apart.