A Glaser Safety Slug works as follows. The bullet consists of a fragile casing (sometimes pre-scored) which contains within it a quantity of lead shot, usually in a suspension of liquid Teflon. The shot and Teflon provide mass for the retention of kinetic energy (momentum); when the bullet strikes a soft surface (such as a person) the energy from the shot is transferred much like a normal bullet since flesh isn't typically solid enough to break the casing. If, however, the bullet strikes a hard surface (such as, in this case, the skin or window of an airplane) the casing breaks, and the kinetic energy carried by the shot and the Teflon is dispersed in all directions. This (hopefully) would be enough to prevent the bullet from penetrating the hardened surface.