Shaft Stiffness & Flexibility

Generally speaking, choosing a racket with the correct shaft flexibility depends on how you swing your racket.

We start from the principle that when a shaft is bent and then released, the stiffer the shaft, the faster it unbends (or straightens or unload...)

When you swing the racket (let's say an overhead clear movement), the racket goes back, and then unleashes forward. At that transitional moment, the shaft bends. How fast you bend the shaft and how you do your forward stroke will determine what shaft you would need.

If a player has a short fast explosive backswing and hit, he'd need a stiffer shaft as it will unload faster into the hitting area.
If that player used a more flexible racket, the shaft will not have time to unbend, so that at impact, the shaft would still be bent backward (ie. instead of doing a smash, he'd be doing a clear...).

On the opposite spectrum, someone with a more smooth slower accelerating stroke would need a more flexible shaft, so that it will unload fully at impact.
If that same player used a more stiffer shaft, the shaft wouldn't have bended as much when the racket went to the back, and also, it woud have straightened prematurely before hitting the shuttle. Therefore he would have lost the benefit of the power generated by a stiffer racket.

In conclusion, the more explosive swing player will benefit more of a stiffer shaft in terms of control and power. The more slower swing player will benefit more from a flexible shaft.

by Loopy @ BadmintonCentral

References:  http://www.badmintoncentral.com/forums/showthread.php?t=40187