In many WWI and WWII movies and shows the soldiers spend a lot of time with their helmets on but not buckled. Sometimes they will do something and then immediately unbuckle it. Maybe everyone in war is much cooler than me, but I would keep that thing buckled at all times, it seems like they would fall off. Did troops really not buckle it or was that just a Hollywood thing to get a better view of the actors face?
There was something almost approaching an urban legend in the WWII US military that if a fastened helmet was hit, a strap would cause whiplash or other injuries.
This previous thread, Did soldiers in WWII really not buckle their helmet chin-straps very often? goes inot this question into more depth.