When a batter begins to make an offering at a pitch but halts his swing before making a genuine attempt, he is said to have produced a checked swing. If the ball does not arrive in the strike zone during such a swing, the pitch is called a "ball". If the ball arrives in the strike zone it is called a "strike", whether or not the batter made a genuine attempt.
A checked swing usually happens when a batter thinks that a given pitch will be to his liking but changes his mind mid-swing. The controversy erupts when the fielding team is sure that the batter has made a definite attempt at the ball and should be called for a strike. If the home plate umpire does not call a strike due to a swing and a miss, the pitcher and/or catcher will often appeal to the first- or third-base umpire (depending on the hitter's stance, right- or left-handed) to determine if the batter did indeed swing at the pitch.
Often the most questioned instance of a checked swing is when a batter turns to avoid a pitch and brings the bat through the zone with no intention of swinging. Such checked swings are often called genuine attempts at the pitch if the bat travels over and across the plate area. The major reason why checked swings produce arguments is because there is no definitive rule for how far the swing has to travel. Traditionally, it is thought to be the halfway point of a swing or entirely across the plate area, past the front of the plate, but this is not codified definitively in baseball rules.