If you read the Web Service documentation you would know that you have to implement at least the
- 'Registering a Device to Receive Push Notifications for a Pass', and
- 'Getting the latest version of a pass'
methods to allow the device to first register with your web service, and then to request the latest version of the pass when the pull to refresh action is triggered.
If you had read the Passbook Programming Guide you would know that each update requires a full .pkpass bundle.
To see exactly what calls the device makes to the web service and the responses it receives, if you have a developer account, enable additional Passbook Logging from your device's developer menu. Then hook your device up to Xcode and watch the console in the organiser as you pull down to refresh.