[MasterSteve]

Operationally, Starbucks packs in a few new features, including a digital tipping service for participating U.S. stores. Because the payment system is handled digitally, customers can tip the Starbucks barista up to two hours after buying a drink. Instead of tipping before tasting their cappuccino, users now have more than enough time to decide if the beverage merits a tip. The system will push out a tip notification after each eligible transaction.

Finally, a new "shake-to-pay" mechanism lets users shake their iPhone to bring up the account barcode needed for scanning at the register. The app must be running for the feature to work.

Source