The purpose is pretty simple. It fetch payment details from gateway side and update local model.

* [PaypalExpressCheckout SyncAction](https://github.com/Payum/Payum/blob/master/src/Payum/Paypal/ExpressCheckout/Nvp/Action/PaymentDetailsSyncAction.php)
* [Payex SyncAction](https://github.com/Payum/Payum/blob/master/src/Payum/Payex/Action/PaymentDetailsSyncAction.php)