We use a PCI compliant payment service for our subscription clients. This service performs a $1.00 auth-charge to verify a credit card if it is changed mid-cycle or after a trial period.
Chargify performs "card validation" when a credit card is added or updated but will not be immediately charged. This occurs, for example, when a free trial is started or when a card is updated mid-period. These validations, which are usually $1.00 auth-only transactions, help ensure that the card can be charged successfully when necessary.