On 3 September 2014 in Class 'E' airspace, a light aircraft carrying out a spin recovery exercise in day VMC came very close to a DHC8 climbing out of Sault Ste. Marie. Although the DH8 crew had seen the light aircraft ahead and above and temporarily levelled off, the light aircraft then began a rapid descending turn and a TCAS RA 'Descend' followed. It was judged that a turn would also be necessary but even with this, the two aircraft subsequently passed within 350-450 feet at the same altitude in opposite directions. The Investigation made no Safety Recommendations.