A card transaction has two parts. When you swipe your card at the time of your purchase, the merchant verifies your account for the amount being purchased and the approved purchase amount is subtracted from your account; this is called a pre-authorization.
The Force Post is the second and final part of the transaction. This occurs when the merchant requests the dollar amount of the purchase and receives the money. Typically, the time span between the pre-auth and the Force Post is one or two business days.