An imperative exclamatory sentence is one that is both an imperative sentence and an exclamatory sentence combined. It has no subject and ends in an exclamation point. Because it expresses strong emotion, it typically expresses a command.

Given the nature of it being a strong command, it isn't unusual for an imperative exclamatory sentence to be very short, even just one word as in: "Stop!"

Be quiet this instant!
Watch out!
Help!