Two related rules govern how a team brings the ball up the floor: the eight-second rule and the backcourt violation, often called over-and-back. Neither is as dramatic as a dunk, but both create turnovers that can swing a game, especially against full-court pressure.
After gaining possession in its backcourt, a team has eight seconds to advance the ball past the midcourt line. The NBA and FIBA both use eight seconds. Before 2000 the international count was ten seconds, and the NBA also used ten seconds before the 2001-02 season. If the offense fails to cross half court in time, the referee calls a violation and the opponent takes the ball out of bounds.
The count runs on the officials' judgment and also links to the shot clock. When the ball is inbounded in the backcourt, the eight-second clock and the shot clock start together. That is why broadcasters watch the shot clock to judge whether a team is about to get called.
The ball counts as being in the frontcourt once it touches the floor or a player who is in the frontcourt. A dribbler has crossed when both feet and the ball are beyond the midcourt line. Until then, the dribbler is still in the backcourt for this purpose, even with one foot over the line.
Once a team has established the ball in the frontcourt, it may not be the first to touch the ball after it returns to the backcourt. If a guard crosses midcourt and then dribbles back over the line, or passes back to a teammate standing behind it, that is a backcourt violation. The ball goes to the defense.
The key word is first. If a defender deflects the ball into the backcourt, the offense may recover it legally because the defense touched it last. Many apparent over-and-back plays are actually legal for this reason, and replays often focus on who touched the ball last.
Several situations are not violations. A player who jumps from the frontcourt, catches a pass in the air, and lands in the backcourt is often protected when the catch comes from a defensive deflection or a throw-in. Rules also give some leeway after a jump ball or during throw-ins. The details differ slightly between leagues, so officials rely on the specific rulebook in use.
Pressing defenses exist partly to exploit these rules. By trapping the ball handler near the sideline before half court, the defense hopes to burn the eight seconds or force a rushed pass. Once the ball crosses midcourt, the defense knows that the half-court line acts like an extra defender, since the offense cannot retreat behind it. Traps right after the crossing are designed to use that.
Offenses counter with quick inbounds, several ball handlers, and timeouts. In the NBA, a timeout called while advancing the ball in the final minutes can let a team inbound in the frontcourt, which skips the eight-second concern altogether.
Backcourt and eight-second violations go into the box score as turnovers for the player or team involved. When you follow a game through a play-by-play feed, these usually appear as a turnover entry with the violation named. If you want to follow NBA games as they happen, the Kadario's NBA scores page page lists live scores so you can see how a late turnover changes a close finish.
Teams get eight seconds to cross half court. Once they have crossed, they cannot be the first to touch the ball back in the backcourt. Defenses design presses around both rules, and offenses design their breakouts to avoid them. Keep those two ideas in mind and most whistles near midcourt make sense.