This is not as often given as a verbal command, as an action that constitutes a command.
When a blanket, or cloak, or covering of any sort, is thrown over the head of a slave, she may not speak or rise. She must remain as she is, still and silent, until the master, or some free free person, lifts the covering away.
The simple command, "Blanket", can also be given, and the slave should behave as though a covering has been thrown over him, remaining this way until commanded otherwise.