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There was a system of hand signals to back up the communication. The movie didn't really show that.

Here's how they did it in Chicago at the Commodities Exchange:

These hand signals are known as arb. I was told (when I worked for a brokerage firm) that the name stems from people involved with arbitrage needing to communicate quickly, before the rest of the market caught on to what they were doing).

https://en.wikipedia.org/wiki/Hand_signaling_(open_outcry)

One of my favorite scenes in Ferris Bueller

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