When a bullet or projectile strikes the body armor, it is caught in a web of high performance fibers that are layered and stitched to exceed certain bullet resistant specifications. The engaged fibers absorb and disperse the impact energy that is transmitted to the vest from the bullet, causing the bullet to deform or "mushroom". Additional energy is absorbed by each successive layer of material in the vest, until such time as the bullet has been stopped. Because the fibers work together in both the individual layer and with other layers of material in the vest, a large area of the garment becomes involved in preventing the bullet from penetrating. Even though the bullet may not penetrate through the body armor, there is still a possibility that the person wearing the body armor may receive injuries to their internal organs. These injuries are a result of blunt trauma force. The body armor provides some protection against these, but it is impossible to prevent it completely especially since it is soft body armor. The reason there are blunt trauma injuries to a person that has been struck by a bullet while wearing body armor is because when the bullet or projectile strikes the vest, there is some initial backward movement by the vest. That means, the bullet does not come to a complete stop upon initial impact yet it comes to stop after a short distance of still going inward towards the body of the person wearing the body armor.

When the bullet strikes the body armor, the body armor moves backwards into the body of the wearer creating a force and the body absorbs some of the force and then acts a barrier to stop the backward movement of the vest. The absorption of the force can cause injuries to the wearer. Those injuries could include anything from bruising to major internal damage that could lead to death. While the number one priority of body armor is to prevent the bullet from penetrating, it is still very important for the body armor to prevent blunt trauma.In regards to hard armor versus soft armor, each type has its own strengths and weaknesses. Hard body armor is made of steel, ceramic or other similar, inflexible materials. Soft body armor is made from Kevlar, Spectra, or similar fabric-like ballistic material. Hard body armor is much stronger than soft body armor - only hard armor is capable of defeating rifle caliber ammunition and some types of armor-piercing ammunition. However, hard armor is typically heavier and is only capable of covering just the vital areas of a person - the heart and lungs, usually.