[SAump]

Batter can't leave the box unless he asks for TIME and PU grants it.

d) The following experimental rule shall be in effect for all National Association Leagues in 2006:
(1) The batter shall keep at least one foot in the batter’s box throughout the batter’s time at bat, unless one of the following exceptions applies, in which case the batter may leave the batter’s box but not the dirt area surrounding home plate:
(i) The batter swings at a pitch;
(ii) The batter is forced out of the batter’s box by a pitch;
(iii) A member of either team requests and is granted “Time”;
(iv) A defensive player attempts a play on a runner at any base;
(v) The batter feints a bunt;
(vi) A wild pitch or passed ball occurs;
(vii) The pitcher leaves the dirt area of the pitching mound after receiving the ball; or
(viii) The catcher leaves the catcher’s box to give defensive signals.
Notwithstanding Rule 6.02(c), if the batter intentionally leaves the batter’s box and delays play, and none of the exceptions listed in Rule 6.02(d)(1)(i) through (viii) applies, the umpire shall award a strike without the pitcher having to deliver the pitch. The ball shall remain alive.

edited to add emphasis