Flexible racquets absorb the centrifugal force, as opposed to transferring this force. Therefore less force is transferred to the ball with flexy frames, than with stiffer frames.
If you take two racquets with the same:
Yet one has a stiffness of 57 (flexible) and the other has a stiffness of 70 (stiff). Swing them at the exact same speed and trajectory at a tennis ball, and you'll find that stiffer of the two racquets will generate a higher velocity ball. This is because more of the force is transferred to the ball.
So whenever you buy flexible racquets, just know that you have to work harder to generate pace. You can combat that by increasing the swing weight. I don't know what the formula is, if you were to take the above experiment and increase the swing weight of the flexible racquet in order to generate the same amount of pace as the stiffer frame. But I'm sure that formula exists somewhere.
perhaps someone around here knows?