The defender gets above his man, between the cutter and the screen he is running toward, and refuses to let him get there. The cutter cannot use the screen because he cannot reach it. Everything that was going to happen off that screen simply does not happen.
Coaches use top-lock on one player, not on everybody. It is a scouting decision about the shooter you have decided to take out of the game.
Why coaches use it
It is the strongest thing you can do to a spot-up shooter who lives off pin-downs and staggers. Chasing him still gives him a catch. Top-locking gives him nothing on that side of the floor and forces the offense to find another entry.
It also puts the defense one pass ahead. If the ball has to go somewhere else, the help is already sitting where it needs to be.
How to attack it
- Go backdoor. A defender standing above you with his back to the rim has given you the whole lane. Cut behind him and ask for it early.
- Flare him. This is the cleanest counter, and it is a called one. The screener turns and sets a flare going the other way, so the defender who was denying the screen is now on the wrong side of a different one.
- Screen his defender. A top-lock defender is shoulder to shoulder with his man and not looking at the floor. He is easy to pin.
- Change the entry. If the shooter cannot get the ball off that screen, put the ball where he is instead.
Need a Three is drawn around exactly this read: run the stagger first, and if the defender tops it, the last screener flares him back to the corner for the same shot.