Whenever he pleases! Unlike the pitcher, the hitter can alternate between the left and right sides of the plate during the same at-bat. The hitter is out if he swaps sides during the windup. However, most batters will stay with one side of the plate throughout the entire at-bat.
In fact, research has shown that hitters tend to get into a groove, where they feel more comfortable batting one way or the other. So by switching sides, the hitter might be trying to confuse the pitcher and wear him down!
Some hitters like to change gears at the beginning of the at-bat to keep the pitcher off-balance. Others will switch directions as many as five times before hitting a ball. It's all part of the learning process and some people call it "fidgeting" with the pitcher. Either way, your coach should know if you are switching sides so she can adjust her approach accordingly.
It's not recommended that you switch sides during an at-bat unless your manager gives you the okay. But if she does let you, then have fun with it!
This week, we'll look at a far more prevalent occurrence: the switch-hitter. With one exception: never during the pitcher's windup. Why is this so? Because by then, the pitcher has decided on a plan of attack and will usually not be distracted from it by changing gears mid-motion.
The exception to this rule comes in the form of leg kickers. These are pitchers who rely primarily on their legs to deliver the ball - sometimes called "steppers" because they will often kick the ground with their heels before throwing. They may do this on either side of the plate, but most tend to favor one over the other. In general, though, they don't like to change directions in the middle of their deliveries - especially from side to side! A good example of this is Randy Johnson - one of the greatest pitchers ever (and one of the few who could beat Mariano Rivera in a head-to-head matchup). You won't see Johnson switch feet very often when he delivers the ball.
Here is where things get a little confusing for young hitters. Since switch-hitters can't legally wear anything other than batting helmets when playing baseball, many young players believe that they must always swing back to the opposite field after hitting into a shift.
The hitter will next decide which side of the plate to bat from. Before any player may "switch," the pitcher must throw one pitch to the hitter. After one pitch, the pitcher and batsman may switch places once every at-bat. If the batter believes that the pitcher is trying to deceive him with a fake pitch, he may protest by saying, "Switch!" The batter cannot protest if he is hitting from the wrong side of the plate.
In addition to deciding how to hit, the batter needs to keep an eye on the ball while it is in flight. This is especially important for left-handed batters who see the ball well before it reaches the plate. They have the ability to stop a runner from advancing too far by simply hitting into a double play. Right-handed batters have this same option if the pitcher starts their delivery with a low blow. They can let the ball go by without batting it if they want to avoid putting the runner out.
In conclusion, switching sides of the plate allows a batter to take advantage of different pitches, field positions, and/or strategies used by the pitcher. It also gives them the opportunity to hit against either side of the infield if necessary.