I don't get the way the FZ28 works with C-AF and Q-AF

I mean: I've read the manual and I know how to set either of them and how they work, Q-AF once the jitter has stopped and C-AF all the time.

The part that I don't get is that, once those pre-focus mode have done their job and the view is in focus, pressing on the shutter button sends the camera hunting for a focus that's already there. What is the use of pre-focussing then ?

The only way around that is to press the MF button but how awkward is this ?

Am I not doing it right ?

Thanks.

Jean