When an offensive sets a screen, the defender [who is blocked] has two choices. They can try to go around, and stay with their man. But that usually enables the offensive player to get separation, because the defender has to get past the screener to stick with their guy. This is possible -- sometimes defenders slide through before the screen gets set cleanly, or they fight their way through despite being "picked."
Or, the defensive player can go "underneath the screen." That is, instead of going around, I cut below the screen and pick up the offensive player on the other side. That usually works best when the offensive player is not looking for a jump shot. Because if you go underneath, the screener creates space for the player you were defending "behind" them, giving them a wide open look.
Now, there are also variations of how to attack it. Players can switch, so the screener's defender can try to pick up on the offensive player who goes around the screen, and the defender who got screened takes the screener [for example].
But the bottom line is that the team is coached to do things a certain way, so that there is no UNCERTAINTY about what to do, whether to switch, etc.
We made a dumb mistake, giving a hot offensive player a wide open three at the end of regulation by blowing how we defended that play.