When the back pressure of a nozzle is below the designed value of pressure at exit of nozzle, the nozzle is said to be

When the back pressure of a nozzle is below the designed value of pressure at the exit of the nozzle, the flow through the nozzle reaches sonic conditions at the throat. This means that the flow velocity at the throat equals the speed of sound. At this point, any further decrease in back pressure will not increase the mass flow rate through the nozzle. This condition is known as choked flow.

In choked flow, the mass flow rate through the nozzle is at its maximum value for the given conditions, and further decreasing the back pressure will not increase the flow rate.

Therefore, when the back pressure of a nozzle is below the designed value of pressure at the exit of the nozzle, the nozzle is said to be choked.