Hydraulic power – The movement of objects using liquids. With the sea, this is the process by which breaking waves compress pockets of air in cracks in a cliff. The pressure may cause the crack to widen, breaking off rock.

To remember the meaning of the term Hydraulic Power, use the following mnemonic:

The water hydrant could flick and power (hydraulic power) away objects using water.

With hydraulic action, cracks in the rock are gradually widened so the waves then compress more air, increasing the explosive force of its release. The effect intensifies in what is known as a "positive feedback" system.

Over time, as the cracks get bigger, they sometimes form caves. The pieces that fall off produce additional types of erosion: abrasion and attrition.