Why Does Flow Become Supersonic in the Diverging Section of a Nozzle?

[ank_gl]

i just started reading about nozzles and turbines in my semester course. i went through converging diverging nozzles. for achieving a supersonic flow, a converging diverging nozzle is used. now a converging section will work as a nozzle only till the flow is subsonic and a diffuser at supersonic flow. that means the maximum velocity obtainable in converging section is speed of sound in the same medium.
also dA/A = [M^2 - 1]dV/V, at Mach 1, section ll not alter the speed of flow.
now consider a converging diverging section. suppose mach 1 is reached at the throat, at mach 1, section ll not alter the speed of flow, then why would the flow become supersonic in the diverging section. i am a little confused about this. since no part of stream can go over mach 1 in converging section, diverging section should act as a diffuser, right?? apparently i am wrong, but i don't know where..
also i was wondering what effect the shape of section might have, i mean concavity or convexity of the nozzle

 

[FredGarvin]

A properly designed CD nozzle will have it's throat at M=1. Compressible theory is where the divergent part starts to work. There are a few funny things that happen because of it. The acceleration of a flow in a divergent section is one of them. Keep reading!

http://exploration.grc.nasa.gov/education/rocket/nozzle.html