The technician isolates the line at a shut-off valve or capped fitting, drains the water, and connects the tracer-gas regulator. Gas is introduced at low pressure — far below the rated working pressure of the pipe — and the system is left to equalise.
The technician then sweeps the surface above the suspected leak path with a calibrated detector. The detector responds to even trace quantities of the gas, so the location of the breach can be pinned to within a few centimetres.