This is covered by:
AQA 8035, Cambridge IGCSE, CEA, Edexcel A, Edexcel B, Eduqas A, OCR A, OCR B, WJEC
Under normal circumstances any cracks or voids in a cliff face will contain air. If a waves hits the cliff face and coveres the entrance to the crack, the air within it will become compressed as the waves tries to force water into it. The sudden compression can weaken the rock around the edges of the crack and cause small bits to break off. As the wave receeds the air is suddenly decompressed and explodes back out of the crack. This also can weaken the surrounding rock and ‘blow’ loosened bits out of the crack. In this way, hydraulic action causes erosion.