When it comes down to hitting, every body wants to hit the ball hard and with power. Of course, the first natural reflex when we need to generate more strength and power is to "flex" our muscles to generate greater forces. It's a normal reflex as more muscles are contracting = more forces being produced.
However, "flexing" your muscles before you hit is counter-productive. Flexed muscles are tensed muscles. Tensed muscles are slow muscles. Slow muscles = slow bat speed. Slow bat speed = reduced power.
What you need to do is stay loose until you are ready to fire. In other words, you stay "loosey-goosey" to use a term I heard Coach Murphy used before until you are ready to attack the ball. Anything you do before should be done while being relaxed. Watch the following video for more explanation about this important physiological principle.