What is hydrostatic testing on a fire extinguisher?

Fire extinguisher contents are maintained under pressure over long periods of time. To ensure that the shell (cylinder) and hose or nozzle has no defects due to metal fatigue or other causes, the cylinders are emptied, filled with water, and then brought up to a specified pressure over a short period of time (typically 30 seconds). The shell (cylinder) must contain that pressure for 1 minute (or the time specified by manufacturer’s maintenance procedures) while the cylinder is inspected for signs of stress, leaks, bulging and other indications of metal fatigue. Hoses on some extinguishers must also be hydrostatically tested.
See the video on YouTube to see a fire extinguisher Completely and Correctly serviced (