The flex is the bend of the shaft. Flexes for Project X clubs are harder than normal shafts.
More weight on the shaft is directed at the bottom of the shaft closer to the head so that more speed can be applied to the swing and more force can be allocated on the ball.
Project X shafts are primarily filled rather than being hollow as with many other shafts.
The shaft helps maintain the stability of the club, not the handle. Because the Project X shafts are more durable, there is more stability.
Because of the harder shafts and the taper distribution, Project X shafts are geared toward low-handicapped players. There is little to no forgiveness with these shafts.