A switch in basketball is the act of a defender switching which offensive player he/she is guarding with one of the defensive player's teammates. Switches are often used by man-to-man defenses in order to counter the screens of an offense.
Whether the offense is using off-ball screens or screens on the ball, the defenders can switch who they are they are guarding. To effectively switch on a screen, the two defensive players involved in the switch need to communicate with each other.
To do this, the defender guarding the screener needs to notify his/her teammate by telling him/her, where the screen is coming from. If one of the defensive players feels he/she will not be able to stick with his/her man after the screen, the defender can call out to switch so both are on the same page.
When an offense employs a number of screens on a possession, the defense might have trouble traversing the floor. It might be difficult, for example, to consistently get over on-ball and off-ball screens in order to defend a good outside shooter. When done correctly, switching can help a defense to perform better as a unit and negate those screens.
It can also cause the offense to play more of a one-on-one game. Rather than penetrating into the gaps of the defense and passing the ball quickly to find an open shot, offensive players will sometimes find themselves isolated against a defender, which can often force a bad shot.
Furthermore, switching is a good way to defend against good three-point shooters. Defenders can usually pressure offensive players on the perimeter without fear of being beaten because of a ball screen. If the offense cannot get past the perimeter, then off-ball defenders do not need to step in and help as much. This can all lead to a contested three-point shot.
As mentioned before, switching can make playing defense against a team that screens heavily more effective. Even if the idea of switching on screens is simple, it can be difficult to execute. Every involved player in defending a screen has to be fully invested in the switch and must switch as soon as the screen occurs.
As such, switching requires players to be vocal and talk about what is happening during the play so their teammates are all on the same page. If a player is unable or unwilling to effectively communicate, the defense will not work properly.
Additionally, switching can cause mismatches to occur. Some defenders are physically capable and skilled enough to guard multiple positions. Many times, however, bigger defenders are too slow or lack the footwork to keep up with a smaller guard. The same goes for smaller defenders, who might not be tall or strong enough to compete with a bigger player near the rim.