There is quite a debate about the Wall Street crash of 1929 and how it relates to the Great Depression that followed, whether it was a failure of government regulation, or part of a natural business cycle that was made worse by New Deal programs. It's mixed in with the debate at present about how much governments should intervene in the market. It is sticky territory to venture into, and it's out of my specialty.

But what you are asking is actually a pretty simple economic question. When a stock market rises, it means value is being put on things like bonds ( loans) and stocks more than currency. When it drops, the reverse is happening: hard currency is becoming more valuable. All this means that the best thing for you to do if you know a real crash is coming in 1929 is to sell your stocks and bonds for hard currency. And put that in a safe place at home and keep it out of a bank, which might take your currency and then fail- this being before the Federal Deposit Insurance Corporation.

You could also short some stocks. That means you borrow them from a broker, sell them immediately at the market price, and when the market drops, buy the same shares at a lower price and give them back to the broker, pocketing the difference.

Charles P. Kindleberger: Manias, Panics and Crashes