Go heavier if a light shaft feels uncontrollable
The fitter explains that a shaft too light for your swing speed turns whippy and uncontrollable - you overpower it and can't control the clubface or sense where it is. Moving to a heavier shaft stabilizes the club through the swing path and helps you catch the sweet spot more often. He uses this to shift the golfer up from a light 95-gram shaft.
Channel: Club Champion
Watch official source at 7:02Why this context matters
The fitter's rationale for moving the golfer off a light 95-gram shaft to a heavier one.