What do you mean auto manual?
Semi auto gearbox or do you want to look at auto and manual gearbox separately?

Oh the system is easy to understand.
There are a few types of semi-auto around. The most simple to understand and make is the AMT used in Savvy(Renault calls it the Quickshift). Basically the clutch is operated robotically. The robot(I only recently knew that some call it the monkey ) disengages the clutch from the engine, while the more traditional way to do that is to use a foot pedal. Since this is a traditional manual you have to release the throttle to shift or the car will jerk.

When you shift to auto mode, the computer will do the thinking for you. The computer communicates with the engine to look at the revs, engine load, car speed and other factors. It decides when to shift, not you.

There are others that are more complex and performs better than the AMT. My favourite is Audi's DSG.

And yes TS, you can google it too.
I'm just explaining here because I like it.

This post has been edited by tunertoobe: Jan 23 2007, 07:52 PM

Howstuffworks.com can explain it in detail.
There are diagrams too.