The idea is for guards inside to be able to shoot out, while attackers can't shoot in. One-way bulletproof glass is usually made of only two layers, a brittle layer on the outside and a flexible one on the inside. A bullet fired from the outside hits the brittle layer first, shattering an area of it. This absorbs some of the bullet's kinetic energy, and spreads it on a larger area. When the slowed bullet hits the flexible layer, it is stopped.