When there is penetrating trauma there is a permanent cavity which is formed by the projectile contacting the tissues. How is a temporary cavity formed?

When a projectile is traveling through space it is giving off energy as it pushes away the air molecules.  When it impacts with human tissue, kinetic energy is transferred from the projectile to the tissue.  When this projectile is a high velocity missile, the tissue which absorbs this energy transfer is often damaged but not disrupted enough to be permanently damaged.