About the film
Signs of the Time is a 60-minute American documentary film on the origin of hand signals in baseball. There are several myths in regard to how signals were started, and the film addresses some of the mysteries that led to umpires giving hand-signals to call plays in the field, base coaches to relay hand signals to players on the field, and catchers to relay hand signals to pitchers.
From the film's Wikipedia article (CC BY-SA 4.0).