77 (because the icon for screen looks like a 7) or Double Drag is an early-offense action that helps to open the middle of the court and let the point guard create for himself or teammates.
A double drag is basically a 3-man screen action. Because three (or more) defenders must move in sync, it’s tough to guard. From the defense’s point of view, the corner help defender (x4) is in a tough spot. He has to decide whether to stay with his matchup or help on the roll. If x4 helps on the roll, there's usually no natural rotation behind him unless the defense communicates early and perfectly. Because of that, the corner shooter is often left wide open.
If the 5 can shoot, the play becomes even more dangerous. The defender has to stay attached to the 5, which creates even more space for the point guard to make plays.
