When a hitter starts toward the middle or back of the batters box then, as the pitch is thrown, starts to creep up toward the pitcher, by the time the pitch reaches the batter she is already moving toward the base.  The hitter "slaps" the ball or hits the ball lightly (almost like a power bunt) just to put it in play.  The "slap" is seen as easier control but with much less power.  If a player has a lot of speed but struggles hitting this is a good way to get her on base.  In softball I believe most slap hitters are left handed since they are already closer to first and do not have to cross the plate to make contact.  The whole point is just to put the ball in play so the player can use their speed to get on base and create some runs.  Even though power is not necessary the ability to concentrate and direct the hit (hitting down the third baseline witll  Ichiro is a good example of a slap hitter in baseball

I think the dimensions facilitate successful slap hitting in softball more so than baseball. The base paths in softball are short enough where just a one or two bouncer can allow a fast runner to reach first safely.