In a few places, you can see traces of this aspect of the system. For example, some gas pumps put a $150 "hold" on your card, and on your bank's web site, you may be able to see "pending" transactions that are in some not-quite-final state.
The nicer feature of preauth is that you can confirm a lower amount. So in the case of a gas pump, they first have to make sure you have the money, without knowing how much, because you haven’t pumped it yet. Once you finish, then they know the amount, and confirm the transaction at a lower amount.