Koivunen was assigned to a ski patrol on 20 April 1944, along with several other men. Three days into their mission, on 18 March, the group was attacked and surrounded by Soviet forces, from which they managed to escape. Koivunen became fatigued after skiing for a long distance, but could not stop. He was also the sole carrier of army-issue Pervitin, or methamphetamine, a stimulant used to remain awake while on duty. Koivunen had trouble pulling out a single pill, so he emptied the entire bottle of thirty capsules into his hand and took them all.
He had a short burst of energy, but then entered into a state of delirium, and lost consciousness. Koivunen remembered waking up the following morning, separated from his patrol and having no supplies. In the following days, he escaped Soviet forces once again, was injured by a land mine, and laid in a ditch for a week waiting for help. After skiing more than 400 km (250 miles) he was found and admitted to a nearby hospital, where his heart rate was measured at 200 beats per minute, double the average human heartbeat, and weighing only 43 kg (94 pounds). In the week Koivunen was gone, he subsisted only on pine buds and a single Siberian jay that he caught and ate raw.
