As soldiers prepared for the war and a battle, they would often have the helmet slinged over the back of their neck until it was time to fight.  While helmets like this weighed around 4-6 pounds, they were light enough not to put too much strain on the neck. When it was time for battle they the would slide the helmet over their face to protect them from the ensuing carnage. With the strap that goes underneath the chin, warriors were able to slip the helmet off their head to the back of their neck when not in combat.  A Greek helmet with no plume often was carried this way.  However other types of helmets such as the Thracian that used two pieces of metal to make the helmet were often larger and more bulky, and were not able to slip off the back of the head.