Margin on a short option is not borrowed money. It is collateral: cash your broker locks up against the worst case of a trade that can lose more than it ever paid you.
The confusion comes from stocks, where margin means a loan to buy more shares. When you sell an option, nothing is lent to you. The broker looks at the position, estimates how badly it could go, and freezes that much of your account as a deposit. The frozen amount is the buying power reduction: money still yours on paper, but you cannot spend it, and it appears nowhere on the trade ticket as a cost.
Buying an option never does this. Pay the premium and your risk is capped at what you paid, so there is nothing extra to lock. Selling is different: the loss is open-ended, so the broker demands standing collateral, and it recalculates the requirement continuously. That last part is where the trouble lives.
The numbers
The stock trades at $100. You sell the $95 put for $2.00 and collect $200 of premium. It feels like free money: the strike sits 5 percent below the price, and the $200 lands in your account the same second.
Now look at your buying power. A typical broker formula locks 20 percent of the stock's value, minus your out-of-the-money cushion, plus the option's current price: $2,000 minus $500 plus $200, about $1,700 of collateral standing behind $200 of premium. You posted eight and a half times the prize.
Then the stock slides to $90. The put is $5 in the money and the position is down about $500 on paper. The broker recalculates: the cushion is gone and the option is worth more, so the requirement climbs to roughly $2,500, up almost half from entry. Your account is shrinking and the lock on it is growing, at the same time, by design.
Where it bites
Sold premium quietly fills the whole tank. Each short put looks small: $200 here, $200 there. But every one locks $1,700 or more, so a handful of "small income trades" can freeze an entire account. When a real opportunity or a real emergency shows up, there is no free capital left; it is all standing guard over premium you already spent.
The requirement grows exactly when you are losing. Margin math feeds on the stock price and on volatility, and a sell-off moves both against you at once: the option goes in the money while volatility spikes, so the broker's worst case gets worse and the lock gets bigger at the precise moment your equity is falling. The point where they cross has a name: the margin call. It never arrives on a calm green day.
Forced liquidation sells your bottom. Miss the call and the broker closes positions for you: market orders, crash-day prices, spreads at their widest. The machine does not wait for the bounce or pick a good exit. And a short put carried to the end has one more door: assignment, where the collateral question turns into owning 100 shares.
Go feel it
Selling premium looks free until the collateral and the tail show up. Sell a put on play money and watch what the "income" actually costs in The Premium Trap.
Related concepts: assignment · bid-ask spread · option premium