The TTL pre-flash just sets the camera's exposure. In an SLR the TTL flash exposure is set by the light reflecting off the film after the mirror is up and the shutter open. This is possible because the shutter speed compared to the flash speed is very long, so no pre-flash is necessary. I assume they can't emulate this system with the Z3 hence the pre-flash. The normal flash method (and cheaper) is not to use TTL but a separate sensor, normally next to the flash head. TTL is the preferred and more accurate.
Concidering the lack of power on the Z3 flash, it seems a bit of a wasted feature for Minolta to put on this model. A bigger flash would have been better.