They wore it on their helmets, carried it around for photos, and, of course, distributed it as a sort of calling card after a combat. The custom was to leave a card in the eyes of the fallen enemy, signifying that the Ace of Spades was the last thing he saw.
The card was flung onto the bodies of dead enemy soldiers as part of psychological warfare designed to reap fear in the Vietnamese. It became evident that the emblem was not employed to gain an advantage over the enemy, but rather to build morale among American forces by assuring them that they were fierce and fearless warriors.