This feature is used by clicking the Authorize Only button after entering the amount you want to authorize.
When successful, the authorized amount appears in the log of payments below (but does not affect the amount actually paid):
To charge the prior authorized amount, click the blue "Charge..." button.
Then verify the amount you want to charge (up the full amount of the previously authorized amount) and click Charge Card.
To cancel a previously authorized amount click the Void link.