On 12 October 2019, an ATR 42-500 on which Captain upgrade line training was being conducted encountered mild clear air turbulence soon after descent began and despite setting flight idle power, a concurrent speed increase led to concern at a possible VMO exceedence. An abrupt and ultimately simultaneous manual increase in pitch attitude followed leading to serious injury to the unsecured cabin crew which rendered them unfit to work. The Investigation found that the upset - a change in pitch from -2.3° to +6.3°in one second - was almost entirely due to pitch input from both pilots rather than turbulence.