Also called a lock-up period. It keeps founders and early investors from selling shares for months to a year or more after an IPO, preventing an immediate flood of selling from crushing the price. During that window, the freely tradable float is small, so the same amount of buying moves the price far more than it would with a normal float. When the lock-up expires and previously restricted shares can trade, the added supply often shakes the stock, so investors track the date closely.