The process can vary a bit according to the vendor, but here's a discussion about how it works for credit unions on the CO-OP network, as an example.

In most cases, debit card authorizations create a Misc. Secured Shares record on the member's account to hold the funds. When the actual transaction comes in, this record is automatically deleted.  (There are exceptions to this rule, such as gas purchases.) 

A little history: In the past, debit card authorizations would add a hold to an independent debit authorization file and put a comment on the account. The hold would delete automatically after two business days during EOD processing, regardless of when the actual transaction was posted to the member's account. That meant that funds might be held longer than actually necessary.  The system is now designed so that funds will be held only until the transaction posted. (If the transaction never actually comes through, the Misc. Secured Funds record will still be deleted after two business days.)

By using a Misc. Secured Funds record, the hold will affect the available balance on the member's account for all activity, not just for other debit card authorizations. That means that everywhere a member's available balance is used (teller posting, Inquiry/Phone, home banking/audio response, share draft posting, ACH, etc.) will reflect the funds being held.  The philosophy is: "If you spend the money, a hold is put on the funds so you can't spend it again anywhere else."

(Remember that in online banking and audio, like everywhere else in CU*BASE, the difference between available balance and actual balance includes things like misc. secured funds, uncollected funds (check holds), and even account freezes. This is explained in detail in the online help available in It's Me 247 online banking.)

CU*BASE has a feature that lets you view a history of activity in the Misc. Secured Funds file, including when the record was created, and when it was purged when the live transaction was actually posted. Use F15-Secure on the Account Inquiry screen, then F18-Sec Funds Hist to access this inquiry. This will help with research should a member question why funds were unavailable at a particular point in time. Just remember also to check Uncollected Funds (check holds) records by using F14-Uncollected on the Account Inquiry screen, then F18-UncHist.

All of our debit card switches are consistent in how they handle authorizations, assuming the authorizations are handled at our end by CU*BASE and not by the switch itself, of course, as may be true in some instances.