The screener starts toward the screen, then leaves for the basket before he ever makes contact. His defender has usually already begun to do his job — stepping out, or preparing to switch — and by the time he looks back, the screener is gone.
Why coaches use it
A slip attacks preparation. The more a defense talks and anticipates, the better it works, which is why teams run it against switching and hedging opponents rather than against a drop.
It also costs nothing to add. Any screen in your playbook can be slipped, and the call is usually one word. Give your screener permission to read it live: if his man leaves early, he goes.
The words are loose here, so agree on them with your staff. A slip comes out of a screen the player intended to set. A ghost screen was never going to be a screen at all, and is usually a guard-to-guard action run specifically at a switching team. Plenty of coaches use the two words interchangeably.
How it is guarded
The screener’s defender has to stay attached and low rather than jumping out early. Against a switching team, the fix is to switch on the first step instead of waiting for contact.
Everything else is a team problem. Somebody has to tag the slipper on his way to the rim, and that somebody is leaving a shooter to do it. Against a good slip your defense is choosing between a layup and a kick-out, which is exactly the choice the offense wanted.
Draw the screen, then drag the screener’s path so he leaves early, and the difference between the two versions is obvious on the replay.