After the fuel is burned and the air is heated, it flows through the last part, the nozzle. The purpose of the nozzle is to turn all of the energy in the air into thrust. In order to do this, the nozzle must first get smaller then get larger. The smallest area of the nozzle is called the "throat". The air coming from the combustor is moving slower than Mach 1, so as the nozzle gets smaller, the air speeds up. The nozzle is designed so that once the air reaches the throat, it has sped all the way up to Mach 1. This is an important detail for the nozzle to work properly. Once the air passes the throat, the air gets larger again. Since the flow reached Mach 1 in the throat, increasing the area also increases the speed of the air. This is the key to creating thrust. The air exiting the nozzle essentially "pushes" the ramjet forward.