What is a drain pipe water drop test?

This Catalyst video shows how a drain (or drainage) pipe water drop test is carried out.

The investigation, which is also known as a drain hydrostatic test, is used to find out if a pipe leaks. It involves sealing the pipe, filling the system with water to a specific pressure, and monitoring the water level over a set period of  time to see if it drops. Water drop tests are often carried out on newly installed pipes or repaired pipes to check that the work carried out has been effective.