The catcher will signal a pitch with their hand between their legs. One finger for a fastball, two for a curve, something like that. The reason this communicati

The catcher will signal a pitch with their hand between their legs. One finger for a fastball, two for a curve, something like that. The reason this communication is necessary is so the catcher sets up correctly to catch e.g. a breaking ball. What the Astros did was zoom in a camera from the outfield to see this and feed that video straight into the dugout. The person in the dugout with the feed would bang a garbage can (or something) to signal audibly to the person up at bat. Knowing what pitch is coming is a massive advantage for a batter. It really has to happen in a matter of a second or two in order to work.