Jim Stone describes how outside blockers can use visual cues to help the middle blocker block the seam shot.
To help block the seam shot, outside blockers must adjust their torso and reach inwards with their hands while in the air. Oftentimes, it's the attacker's shoulder that gives away the seam ball. That means the outside blocker has to watch for the attacker's arm swing and adjust after she has jumped vertically.
