This is a vertical STS (Screen-the-Screener) play. First screen creates confusion. Second screen creates the finish.
The key: You are not freeing the shooter. You are setting up the screener.
Screen-the-Screener actions are deadly because they punish the defense for communicating correctly on the first action. By the time the defense successfully navigates the initial down screen, the blind-side, back-screen has already destroyed their positioning.