Heterodon platirhinos runs a two-stage defence. First it inflates the body, flattens the neck into a hood, hisses loudly and makes bluff strikes with a closed mouth. The display is convincing enough that hognoses are frequently killed as cobras or vipers despite being effectively harmless.
If the bluff fails, the snake switches to death feigning, or thanatosis. It thrashes, opens the mouth, lets the tongue hang out, often smears foul-smelling cloacal fluid, and settles upside down. Experimental work has shown the display is not a reflex loss of control: the snake breaks the pose faster when the observer looks away, and studies have recorded hognoses watching the threat during the act.
The tell is the flip. A hognose turned right side up will immediately roll back onto its back, because being ventral-side-up appears to be part of the fixed script rather than a consequence of collapse. The behaviour probably works against predators that reject carrion or that lose interest in motionless prey.