Hydraulic action is a form of coastal erosion. It happens when the waves break against the cliffs and over long periods of time it makes pieces of the cliff break off.
Hydraulic action happens when the waves break against the cliff. Air is trapped between the sea water and the cliff. The wave keeps going and forces the compressed air into cracks in the cliff.
Waves continue hitting the cliff and forcing air into cracks in the cliff. Eventually, this begins to enlarge the crack and erode the rock.