You duck behind something, using it to take cover against attacks. If you perform this action as a Reaction in response to an attack, you perform it before you make any saving throws against that attack.
If you are not gaining any kind of Cover from an adjacent object or obstruction, but it is large enough to cover half your body if you crouch behind it, you gain Half Cover until you use a Movement or non-Curse Attack action.
If you are already gaining Half Cover from an object or obstruction without using this action, you now gain Full Cover until you use a Movement or non-Curse Attack action.
If you are already gaining Full Cover or better from the object or obstruction, you can use this action to lean out from behind it and use your remaining actions to make an attack against something that you would normally be unable to target from your position, effectively using a square adjacent to yours as the origin of the attack without putting yourself at additional risk.