I answered a similar question to this here but will repost it below. The commander of the 29th Infantry Division, Major General Charles H. Gerhardt, was a notorious stickler to the rules, and preferred that the chinstrap be fastened at all times.

It was common for orders to be issued whereby the helmet was to be worn with the chin strap securely fastened either under the chin or on the point of the chin at all times, either out of neatness (even when not on parade) or to avoid losing the helmet in battle, exposing the soldier to the potential of (perceived needless) injury or death. One reason that was given for soldiers to not wear their helmets with the chin straps fastened was that the concussive blast from a nearby explosion would "catch" on the rim of the helmet and jerk their heads back violently, breaking their necks and killing them; this was recognized as a myth even during the war, but it still persists today for some reason.

There was a twinkle in the eyes of some when they heard of the instructions which had come from First Army concerning helmet chin-straps; they were to be put up over the back of the helmet and never worn fastened under the chin. (In this Regiment it had been one of those unpardonable breeches [sic] of discipline to be seen with chinstraps not properly fastened). Theoretically this order had been originated in order to avoid broken necks resulting from the sudden upward jerk of helmets when the concussion of near bomb or shell hit it. Actually, no such case has ever been authenticated, and it is quite likely that more serious casualties resulted from loss of the helmet at a critical moment than would have from any such effects of the chinstrap.

It was so pervasive that the Army developed a new chinstrap for the M1 helmet in September 1944, the T1, which was designed to automatically unhook if fixed in the proper position and subject to a blast pressure of more than 15 pounds per square inch. This would likely result in burst eardrums, and approaches the threshold for lung damage and it's likely that men close enough to an explosion to have their chinstraps unhook as designed would otherwise be killed anyway for other reasons.

The 90th Infantry Division Preservation Group analyzed 2,500 photos of American troops from the Normandy campaign, and selected 90 photos that were felt to be representative of the conditions for all troops during that period and where the use of one or multiple M1 helmets could be clearly discerned. 247 helmets were analyzed.

Chinstrap state	Number	Percentage of total
Up, and affixed across the back (nape) of the helmet	119	48.18%
Chinstraps not visible or not identifiable	64	25.91%
Dangling down at the sides of the helmet	15	6.07%
Fastened underneath the chin as designed	15	6.07%
Up, attached/woven into a helmet net	14	5.67%
Placed in the back (nape) of the helmet, between the shell and liner	5	2.02%
Entirely missing, torn, or cut off	3	1.21%