A look at how borrowed money can turn a normal stock sell-off into a self-feeding crash — bad news for anyone trading on margin.

• When people buy stocks with borrowed money, a falling price can force sales automatically, no willing seller needed.
• If your stocks drop enough, the broker demands more cash fast, and if you can't pay, they dump your shares at any price.
• Each round of forced selling pushes prices lower, which triggers the next round of margin calls — a doom loop.
• In Korea's recent crash, forced sell-offs hit five times the normal rate, and leveraged ETFs made the spiral worse.

Outlook: As long as markets run on heavy borrowed money, a sharp drop can snowball into a much bigger crash than the original selling would suggest.