Base bleed is a method of extending the range of artillery (or naval gunfire) shells. The low-pressure area immediately behind the shell is a primary source of drag during the shell's flight. A base bleed projectile's casing has been designed to direct airflow smoothly into this low-pressure area, usually through slots and grooves designed to redirect the airflow without turbulence. This increased airflow into the low-pressure area lowers the pressure differential between the front and back of the shell, allowing it to travel further. This is a purely passive mechanism that does not require any additional pyrotechnics or systems on the shell, which is an advantage; however, the range extension is not as great as can be achieved with base burn or base boost projectiles.