You drop to the ground, becoming Prone.
You can use this action as a Reaction at any time, including immediately before you make a saving throw against an attack. You can choose to use a Free Action instead of a Reaction to do this, but when you do so you gain +1 level of the Stunned bane due to how fast and uncontrolled your drop was.
This action does not trigger Reactions, such as Attack of Opportunity.