The most important shaft fitting characteristic is the golfer’s point of wrist cock release on the downswing. This is not to minimize the importance of the golfer’s swing speed, transition and tempo play in the selection of the right shaft for a golfer. But the point at which the wrist cock angle is unhinged on the downswing is the one swing factor that determines whether the shaft’s stiffness design will be a performance element in the shot or not.
To comprehend this requires a basic understanding of how the shaft bends during the downswing. The shaft begins to bend in response to the force of the transition move that initiates the downswing. The moment the golfer begins to unhinge their wrist cock angle, the shaft now begins to reverse its bending action. As the clubhead accelerates in response to the unhinging of the wrist cock angle, the clubhead drives the shaft to bend forward. If the golfer unhinges the wrist cock angle early on the downswing, the forward bending action of tha shaft occurs well before impact so by the time the club gets to the ball, the shaft has rebounded back and arrives at impact in a straight position. However, for player with a late release, the shaft arrives at impact flexed forward slightly, in turn altering the loft on the clubhead at the actual moment of impact. This is how a shaft can be designed to bring about a lower, medium or higher trajectory.
For a golfer with a late release, the stiffer the shaft and/or the more tip stiff the shaft, the lower the ball flight will be because the shaft is not flexed forward as much. Vice versa, for such a later release golfer, the more flexible the shaft and the more tip flexible the design of the shaft, the higher the flight of the ball will be because the shaft is flexed forward more.
For the early release golfer, differences in the flex design of the shaft do not result in differences in the height of the shot because the shaft cannot arrive at impact in a flexed forward position. As a result, only for the later to very late release golfer will the shaft’s stiffness design be a visible contributor to the height and the amount of backspin on the shot. This is why I consider the point of the golfer’s wrist cock release as the single most important shaft fitting characteristic.
Second to this is the forcefulness of the golfer’s transition move to start the downswing. The more aggressive the golfer starts the downswing, the more the golfer exerts a bending action on the shaft and from that, the stiffer the shaft selection for the golfer should be.