
Blacky, for a shaft to work at max efficiency and generate distance the golfer has to "load" it....i.e. feed it some energy that can be transmitted to the ball at impact.
Imagine stretching a rubber band...by doing this you are storing enrgy in the band, when you let go the energy is released.
In golf a similar thing is done thru the backswing and the transition into and during the downswing. Becasue there is a heavy weight on the end of the shaft (i.e. the club head), when you swing the club the shaft will bend away from its natural straight shape...energy is now stored in the shaft or..."it is loaded". How aggressively you swing will have a direct influence upon how you "load" the shaft. Fast swingers who have a quick transition from backswing to downswing exert a heavy load on the shaft...hence they generally require stiffer shafts.
During the downswing the shaft will naturally spring back to its natural straight shape and hence transfer the stored energy to the ball.
The tricky thing in choosing a shaft is to get the balance right between the ease of loading of the shaft and the control you have of the ball. A very flexible shaft will be difficult to control where it releases the energy and will generally spray the ball all over the place. Too stiff a shaft will usually hit the ball in a straight line determined largely by the club swing path but will not give great distance becasue the golfer couldnt load it properly.