The airplane experienced an abrupt maneuver during initial descent and a flight attendant fractured their ankle. According to the flight crew, just prior to top of descent the captain informed the flight attendants to prepare the cabin early because there was weather in the vicinity of the arrival airport. Shortly after that, the first officer (FO), who was the pilot flying, initiated the descent. Air traffic control then cleared the flight to a lower altitude and requested they increase their descent. The FO complied, and increased their vertical speed, however, at some point, he engaged "open descent" mode. The FO saw the airspeed increasing towards red line and he pulled back on the stick and the autopilot disengaged. The captain said he then took control using the autopilot disconnect/priority switch and applied aft stick pressure to return the airplane to a normal descent pitch attitude to prevent an overspeed. According to the flight crew, the airplane was in clear air and smooth flight conditions when the event occurred. Two flight attendants (FA) in the aft galley, were thrown into the air and onto the floor. The captain was notified that one of the FAs had been injured and would need assistance when they got to the gate. A deadheading FA assumed the injured FAs duties for the remainder of the flight. After landing, the FA was transported to a local hospital and diagnosed with a fractured ankle.