These men were in a Serbian dugout on the side of a mountain, cleverly screened as well as protected by heavy stones upon the roof. An observer in an enemy plane would not be apt to suspect the existence of this shelter. A telephone wire ran from this shelter to a listening post nearer the front, on the edge of "No Man's Land" in fact. At the listening post a sentinel was keenly alert. Ofttimes the movements of the enemy were concealed by fog. Often he attacked at night. At such times, when nothing could be seen, an alert soldier at the listening post could often determine when an attack was coming by the scuffle of advancing feet, the rolling of a displaced stone, the tinkling of empty cans hung on the barb wire which always stretched in front of the trenches as a screen. The moment suspicious sounds broke upon his ear, he phoned. Instantly these men, lolling at ease before us, sprang to action. The rocket was fired, spreading over "No Man's Land" as it slowly settled down, a brilliant glare sometimes lasting three or four minutes, and bringing into bold relief not only the attacking party but every stick and stone as well. The defenders of the position then opened on the attacking force with machine guns, grenades and rifle fire. Soldiers were trained to "Frencheze" as soon as a glare lighted up the field, that is, to remain perfectly still in whatever position they happened to be, on the theory, demonstrated by experience, that a motionless body was more likely to escape detection than one in action.
Keystone View Company, “Serbian trench - Awaiting phone call from listening post to fire rocket for illuminating "No Man's Land",” Monash Collections Online, accessed September 21, 2020, http://repository.monash.edu/items/show/25413