A tradition in some US military units wherein a soldier, airman, sailor or Marine being awarded a medal featuring wings (e.g. Airborne, pilot/aviator/aircrew, Pathfinder). A superior will place the medal in the appropriate location of the recipient's uniform but not attach the clasps on the back of the pins. The superior (or another person who has already received the designation being awarded) strikes the recipient in the chest, causing the pins to be pressed into the wearer's skin -- inevitably leading to pain and blood.
Following the 1997 release of a videotape depicting one of these "ceremonies," most of the branches of the Department of Defense cracked down heavily on such practices. The video featured a group of Marines hazing 10 new graduates of the Army Airborne School at Fort Benning, Georgia. The actions by the Marines vastly exceeded the "typical" hazings and became a public relations nightmare for many officers involved with jump school and others throughout the military.
Today, blood winging continues to occur; those involved try to keep it under the radar.