Sonic airflow though divergent and convergent ducts
When a flow of fluid (i.e. gas) flows at sonic speed through a convergent duct a shock wave forms at the exit area of the duct - The exit area is said to be choked. The shock wave forms a restriction to the fluid and pressure will increase, temperature will increase and velocity will decrease.
When a gas flow reaches sonic velocity in a convergent duct the nozzle will choke and the pressure will increase. To prevent a pressure rise that would eventually prevent a 'fluid' flow and completely choke the duct a divergent section is added making the duct convergent/divergent (Con/DI). The pressure of gas released into the divergent section of the nozzle causes the velocity of the 'fluid' to increase, pressure to decrease, and therefore temperature to decrease. Gas pressure acts on the walls of the divergent section, this pressure gives additional thrust that is known as pressure thrust.