PC engine is not started by a normal 12V starter, instead the engine is started by one of the EV motors through the power split device:
http://auto.howstuffworks.com/hybrid-car7.htm
http://eahart.com/prius/psd/

Why PC can save more in a traffic jam:
1) When you arrive at a traffic jam from a higher speed, you will slow down and PC will capture some of the energy back into the battery. Normally the battery will be more than half full upon reaching the jam.

2) Depending on how long is the traffic jam, sometimes the battery charge is enough to get you crawling through the traffic without starting the engine. This will contribute a major fuel saving compared to leaving the engine idle.

3) Even when the jam is longer, the car will only start its engine and runs it at the most efficient speed to charge the battery and after a while it will stop again. This is much more efficient than an idling engine doing nothing but powering the air cond compressor for a long time.

Your air cond at home only needs 1hp to cool the entire room but letting your engine idle to produce more power than neccessary is a waste of energy. You would think that the extra power is not that much but if you release the brakes, you know that the engine is actually producing enough power to move the car at a crawling speed without even pressing the throttle.

The PC on the other hand only starts the engine to generate the required electricity. After that it will shut off again. The stored energy is then used to power the aircond and it will only be used to move the car when required. Compared this to an idling engine that is producing enough power for the aircond and to move the car all the time while in a traffic jam.