Authorisations are done in real time. This is where the funds are checked to see if they're available, and if so, they are reserved ("ring fenced") in your account for a period of time. Debits are a different thing, and they get sent through to the bank in batches, usually once a day overnight, and the bank then takes a few days to actually move the money around.
That's how it works with an Internet merchant account at least. I don't know if it's the same for a PDQ machine.