Golf shafts twist during the swing. The higher number is a greater rotation with the same force. A higher torque for a slower swing will help "snap" the head through the ball. With a higher swing speed, the head position will be unstable at the point of contact. It's not simply a function of club head speed. The quicker transition, the more torque gets applied by the golfer. That's why 2 golfers at the same club head speed may both use the same shaft for launch but different flexes based on differences in transition.

Don is right especially about the transition. I swing anywhere between 92-95 mph, but very smooth due to a disintegrating lower L5 disc in my back. When I was fit at the Titleist Thursday’s, I tried both the stiff 6.0 and regular 5.5 Hzrdus Smoke and the fitter recommended me to go with the 5.5 with my smooth transition. He said to error on the side of the softer flex if you are in between flexes. The 5.5 was 3 yards longer carry and tighter dispersion than the 6.0 with a baby draw instead of a slight push.

I was fitted for my 917 from the same fitter and gained 4 mph ball speed, 20 yards carry and lowered my spin by 400 rpm with my new TS2 9.5 B2 setting in a 5.5 Hzrdus Smoke at 45.5 inches. Very happy!!