Instead of fighting over the top of the screen, the on-ball defender slides underneath it, between the screener and his own big, and arrives on the far side before the dribbler gets there. He never loses contact with the drive. What he gives up is the shot.
Going under is a decision about one player. It says, in public, that you do not believe this handler will make the pull-up.
Why coaches use it
It is the cheapest way to stay in front of a fast guard. Nobody has to hedge, nobody has to switch, and the big can stay home in a drop, so the rim is protected and no shooter is left alone. Against a non-shooting point guard it turns a pick and roll into a bad two.
Against a shooting guard it is a gift, and everyone in the gym can see it.
How to attack it
- Shoot it. That is the answer the coverage asked for. Make two and they will stop going under.
- Re-screen. Screen him again the moment he comes out the other side. Horns Twist is built on exactly this idea: the second screen catches a defender who is still recovering from the first.
- Move the screen out. A screen set five feet further from the rim makes the under path that much longer and the resulting shot that much shorter.
- Pop the screener. A big who steps behind the arc punishes the drop that usually comes with an under.
- Snake it. Keep the dribble alive back toward the middle and the defender who went under has to change direction twice.
Note that “under” and “over” describe the guard’s path only. What the big does behind him is a separate choice, which is why most staffs call the coverage by both halves.