Sterling fell several percent against the dollar in a matter of minutes during Asian hours in October 2016, and recovered most of it before London opened. The yen did something similar in early January 2019, in the thin session between the New Year holidays, with several crosses printing prices that traded nowhere else. Both events shared a shape rather than a cause: an order book that emptied faster than anyone could react.
The mechanics are worth understanding precisely, because the popular explanation, that an algorithm went rogue and sold, is usually backwards.
Books empty faster than they fill
Modern market making is automated. An algorithm posts a two sided quote and manages the risk it accumulates. Every one of those systems has conditions under which it stops quoting: volatility above a threshold, a stale reference price, a latency spike, an inventory limit reached. Those conditions are similar across firms because the risks being managed are similar.
So when volatility jumps, liquidity providers do not compete harder for flow. Many withdraw at once. The book that showed continuous depth a second earlier has holes in it. Any order arriving into those holes walks through multiple price levels, which raises measured volatility, which trips more withdrawal thresholds. That feedback loop is the crash.
The recovery follows from the same logic. Once the initial flow is absorbed and volatility readings settle, the quoting systems come back, the book refills, and price returns toward where it was. This is why flash crashes retrace so much: nothing fundamental was decided, the price simply had nowhere to rest for a few minutes.
Stop cascades add the fuel
Stop loss orders are conditional market orders. They do nothing until a level trades, then they all become sell orders into a book that is already thin. Because retail and institutional stops cluster below round numbers and recent lows, the first break triggers a batch, which pushes price into the next batch.
Margin liquidations behave the same way and are worse, because a broker's risk system closing out an under margined account is indifferent to price. It has to trade. A cascade of forced closes is the reason a move that started as a 40 pip dislocation can extend into several hundred.
The lesson is not to trade without stops. It is that a stop guarantees an exit attempt, not an exit price. Our guide to stop loss placement covers the trade off between a stop close enough to limit loss and one far enough from the obvious cluster.
In a flash crash the fill you receive can be far from the level you set. That is not a broker error, it is what a market order does when there is no bid at your price. Negative balance protection, where it applies, limits how far past zero the damage can go.
Why they cluster in the quiet hours
Both of the events above happened during Asian hours. That is not coincidence. The book is thinnest when the major centres are closed, so the amount of flow required to start the loop is far smaller. Add a public holiday and the threshold drops again, which is the argument in our piece on holiday liquidity.
Equity and futures markets have circuit breakers and limit up limit down bands that halt trading when price moves too far too quickly, giving the book time to refill. Spot FX has no central exchange and therefore no equivalent halt. Individual venues may pause, but the market as a whole cannot be stopped, which is one reason FX dislocations can be more extreme in percentage terms than a comparable equity index event.
What a trader can actually control
Nothing about the timing. Everything about the exposure.
Position size is the only real lever, because it determines how much a gap of unknown size costs you. A trader running 1% risk per trade on a normal stop has a very different outcome to one running 8%, when the fill comes 300 pips past the stop. That arithmetic is the subject of our risk management rules, and flash crashes are the scenario those rules exist for.
Overnight and weekend exposure through thin sessions is the next lever. Then account level protections: check whether the firm applies negative balance protection to your account type and jurisdiction rather than assuming it, since the rule is a regulatory requirement in some places and a commercial policy in others.
Guaranteed stops, where a firm offers them, remove the fill uncertainty in exchange for a premium or a wider spread. They are the only instrument that actually converts gap risk into a known cost, and they are worth understanding even if you decide the price is not worth paying.
Trading into one is usually a mistake
The reversal is visible in hindsight and looks like an obvious opportunity. Live, it is not. Spreads during the event can be tens or hundreds of times normal, so an entry taken at the extreme carries a cost that eats most of the retrace. Platforms may reject or requote orders because the price they quoted is already gone, behaviour covered in our note on requotes and execution. And some events do not retrace at all, because the dislocation coincided with real news.
The professional response is duller than the war stories suggest: stand aside, let the book refill, and look at the chart afterwards to decide whether anything about your view changed. Leveraged trading carries a high risk of loss in ordinary conditions, and a liquidity vacuum is where that risk stops being theoretical.
"You cannot forecast a flash crash. You can decide in advance how much of your account is allowed to be exposed when one happens, and that decision is the only defence that has ever worked."
— Alex Onta, Executive Director, SINGUARD
Key Takeaways
- A flash crash is caused by liquidity providers withdrawing quotes at once, not by one large seller.
- Stop clusters and forced margin closes turn a small dislocation into a cascade, because both are price indifferent orders.
- Thin sessions and holidays lower the amount of flow needed to start the loop, which is why so many events occur in Asian hours.
- Position size and overnight exposure are the only levers a trader controls; guaranteed stops convert gap risk into a known cost.
Frequently Asked Questions
What causes a flash crash?
Automated market makers withdraw their quotes when volatility, latency or inventory limits are breached. Because those thresholds are similar across firms, many withdraw at the same moment, leaving gaps in the order book. Orders arriving into the gaps move price sharply, which raises volatility and triggers more withdrawals.
Why do flash crashes reverse so quickly?
Because nothing fundamental was resolved. Once the initial flow is absorbed and volatility readings normalise, the quoting systems return, the book refills and price moves back toward where it was trading before the vacuum opened.
Can a stop loss protect me in a flash crash?
A stop loss triggers an exit attempt at the market once your level trades, but it does not guarantee the price. In a thin book the fill can be well beyond the stop level. A guaranteed stop, where a firm offers one, does fix the exit price in exchange for a premium or a wider spread.
About the Author
Alex Onta is an Executive Director at SINGUARD. He built eTrader, the terminal, the mobile apps, eTrader Broker, Copytrading, Business and Community, along with the worldwide clustered-server infrastructure it all runs on, with his brother Roman Onta helping on the design, and he leads that division today. Together with Roman he builds the Prop Firm CRM, the Broker CRM, Scalegram and CopySignals, and the two of them carry worldwide compliance, payment processing and international business structuring side by side. He lives and works in Dubai for most of the year. Meet the executive duo leading Singuard's five divisions.