This is coolbert:
From that era of the Second World War, Korea and Vietnam the American M1 helmet as worn by the soldiers of those periods, some extracts regarding same as found in the most recent issue of WW2 magazine. Myths repeated and exposed.
The truth is out there! Thank you WW2 magazine.
Q: "Why didn't U.S. Army and Marine infantry connect their helmet chinstraps when under fire? Newsreels and even Hollywood movies show chinstraps dangling on the side of heads or connected around the rear of the helmet."
A: "Men had two reasons - - one valid, one specious - - to leave helmet straps loose or clip them behind. A man strapped into his steel bucket risked an attacker coming from behind grabbing his helmet visor to thrown him off balance. men also believed wrongly - - that if a shell or bomb went off nearby, a tightly fastened helmet would snap its wearer's neck."
This too is what I was told in basic training having to wear the M1 helmet the entire time. That helmet consisting of two parts, the steel pot outer element and the inside helmet liner [plastic]. NOT comfortable to wear and yes indeed, "infantry advancing with one had holding helmets down and the other holding their weapon" was exactly the case. That helmet uncomfortable and awkward, bouncing around as you ran!
Personally, that an enemy soldier would be able to approach you from behind and throw you off balance by grabbing the visor seems like a "specious" reason also. What troop allows the enemy to approach you from behind? Always keep your foe to the front!
Frankly too I would expect the danger from splinters of a nearby detonating enemy shell would pose a much greater danger to your body and being than having the neck snapped from the detonation. But what do I know?