You tell it very well!
The nozzle pressure ratio is the ratio between the exhaust pressure and the external pressure. In normal conditions the nozzle is adapted to the external pressure, so the pressure ratio is one, but when the pilot use the afterburner, the nozzle opens up, making those characteristic disks, called Mach disks, a shock wave recompression. I found this in the web: http://www.aerospaceweb.org/question/propulsion/q0224.shtml