On 31 July 2021, a Boeing 737-800 descending through an area of convective activity which was subject to a current SIGMET encountered some anticipated moderate turbulence whilst visually deviating around storm cells without reducing speed. When it appeared possible that the maximum speed may be exceeded because of turbulence, the autopilot was disconnected and a severe pitch up and then down immediately followed resulting in serious injuries to two of the four cabin crew and a passenger. This disconnection was contrary to the aircraft operator’s procedures and to the explicit training received by the pilot involved who was in command.