As far as the physics to this, I'll let someone else explain
From what I understand though the "flex" of a bat gives you a bit of a 'trampoline' effect to sort of launch the ball. However too much flex and you have too much give (backwards) as you are trying to (instead) drive the ball forward.
As far as the Easton bats, the explanation I've been given is that the "regular" flex bat is desgined more for a "singles" type hitter and the "stiff" is designed more for a "power" type hitter. Not sure about high school players, but I've only seen the "stiff" used at the college level. My son used the '05 stiff and loved it and his school tested the new '06 stiff and said it was an excellent bat.