If the price of a share of stock is falling, an investor is unlikely to be able to execute an at-or-better trade because buyers are more likely to wait in order to obtain a lower price.
For example, if a share is trading at $10, a trader can put in an at-or-better order to sell the security at $10.10. Any price greater than or equal to $10.10 will trigger a sale.
Investment dictionary. Academic. 2012.