This video shows the basics of fluid cavitation, including demonstration from AvE. If you subject a fluid to a sudden change in pressure, some interesting things can happen. You can cause tremendous damage to moving parts, or you can harness this destructive power in many beneficial ways.
Water hammer can work in both directions, and I only discussed one of those in the previous video (https://youtu.be/xoLmVFAFjn4). This episode revisits that demonstration to show how water hammer can form a vacuum pressure in a pipe. Momentum carrying fluid away from a valve wants to keep going even after the valve is closed. This generates a negative pressure than can cause major damage!