When you add your card to Curve, we send to preauthorisations of £1 to authenticate the card. These are authorisations only, and the money will never leave your account.
When you tap 'verify card' we send a code to your bank statement. In order to do this we have to charge your account a small amount (less than £1.50) which is then refunded immediately. The refund may take 2-3 working days to display on your statement.