Until several years ago, this method of tank cycling was the only way to establish a saltwater aquarium. Initially the process involved setting a tank up with dry ocean rock (also known as aragonite).
Ammonia was introduced to the tank by adding a prawn, or a muscle, or even ammonia and patiently waiting for nitrifying bacteria to populate the tank.
Through testing the water you could establish at what stage the nitrifying bacteria where at. If a test came back with high ammonia, then you knew that the nitrosomanas bacteria had not yet successfully established themselves.
When ammonia was diminishing or gone altogether, but nitrite was evident, the nitrosomanas bacteria where pretty well established, converting ammonia into nitrites.
When nitrates were being detected, you were confident that the nitrobacter bacteria were now converting nitrites to nitrate.