Because all shafts are larger at the grip end and taper down to be smaller at the head end, that means the tip end is the weakest side and the grip end is the strongest side of the shaft. Therefore, cutting more of the tip will have the effect of getting rid of some of the weaker end of the shaft which, in turn, makes the shaft play and feel more stiff. Cutting more of the grip end will still stiffen the shaft a little bit, but only because in doing so you make the shaft shorter, and not nearly as much as when trimming more from the tip end.
How much any increment of additional trimming changes the flex of the shaft is completely individual and determined by the original design of the shaft. There are some shafts in which trimming an additional one inch from the tip will change the stiffness barely at all, while in other shaft designs a 1-inch additional cut from the tip end will increase the stiffness quite noticeably.
