What a crash actually is
A crash is a repricing, usually of everything at once, driven by fear feeding on itself. The businesses underneath mostly keep operating: groceries get sold, software subscriptions renew, dividends (mostly) get paid. What changed violently is the price other people will pay today, which only matters if you must sell today.
That is why the emergency fund is portfolio strategy: it is the thing that makes you a person who does not have to sell today.
Why selling feels right and is usually wrong
In a crash your brain does exactly what it evolved to do: treat loss as a predator and run. Selling delivers instant relief, which is why so many people do it at the bottom. The cost arrives later: the best days in market history sit clustered inside the worst periods, and missing a handful of them cuts decades of returns dramatically. Getting out requires being right twice, on the exit and on the re-entry, and the second decision is even harder because it never feels safe until prices have already recovered.
What preparation looks like
Three things, all done in calm weather. An emergency fund so the portfolio never pays the bills. An allocation you can actually sleep through, which is discovered honestly by asking what a 40% fall in your equity sleeve does to your behaviour, not your spreadsheet. And a written plan: the market-crash checklist on this site exists precisely so panicking-you takes instructions from calm-you.
Then, in the crash itself: keep the direct debit running (you are buying the sale), check prices weekly not hourly, and re-read individual theses rather than selling the market wholesale. Crashes transfer wealth from the people who must act to the people who planned not to.