Homework / test English Other General / Conversation / Greetings / Letters London, circa 1900

From Hound of the Baskervilles:

"Until we got three-quarters down Regent Street. Then my gentleman threw up the trap, and he cried that I should drive right away to Waterloo Station as hard as I could go."

I'm assuming this refers to the same thing as in the previous chapter:

"Instantly the trapdoor at the top flew up, something was screamed to the driver..."

But I've never seen an English horse-drawn cab, and I have trouble visualizing what exactly this is or what signal this action sends.