A short sale restriction (SSR) triggers automatically when a stock falls 10% or more from the previous session's closing price, activating SEC Rule 201. Once triggered, the SSR remains in effect for the rest of the trading day and the following trading day.
This means that traders can only execute a short sale at a price higher than the current highest bid, preventing them from contributing to the stock's decline.
The rule, also called the alternative uptick rule, exists to prevent short sellers from accelerating a freefall. You can still short, but you must wait for an uptick, which often fuels squeezes in already volatile stocks.