Envoy flight 4265 experienced a jolt during pushback and a flight attendant broke a wrist. According to the captain, after the “Before Start Checklist” was completed, the flightcrew was connected to the ground crew push-tug operator for push-back via the interphone. The push-tug operator informed him that he was ready for the captain to release the parking brake, and upon the captain doing so, the push began. During the final push, when the airplane transitioned from reverse to forward, the captain said that the communications connection with the ground crew deteriorated, and he believed he heard the push-tug operator say “set parking brake”, when actually the push-tug operator was directing the flight crew to start engines. The captain said that as a result he applied the parking brake while the tug operator was pulling the airplane forward, which resulted in a sudden jerk. The captain also said that the push-tug operator was difficult to understand because of his accent, which combined with the faulty interphone connection, resulted in the miscommunication. At the time of the event, two flight attendants (FA) were in the aisle performing the passenger safety demonstration. No. 1 FA was thrown against seat 4D, and No. 2 FA was thrown to the floor, hitting her head, back, and wrist, and was unable to stand. The airplane was towed back to the gate, and the No. 2 FA was taken to a local hospital where she was diagnosed with a fractured left wrist.