With some engines, the water is injected as the starter is run. With others it's injected while the engine is running somewhere above idle. See the engine manufacturer's recommendations for this.
What is the cleaning agent.
Water. Preferably processed by reverse osmosis, or distilled.
Is a special port used for the infusion/spray.
Yes
Does this port get a wire through the fitting to prevent it loosening.
It's a tube, with a connector to the wash bottle on one end, and the other end is pointed into the air intake, in most cases. No wires, it's held in place by clamps.
Would domestic owners be allowed to do their own, or is it the sole preserve of the engineers.
I suspect that depends on the country and its authorities.
Please outline the negative aspects of this procedure.
I don't know of any.
By what % could the quality of an engine be maintained and is the procedure related to the hours flown & conditions of the environment.
To the environment, certainly. In the Gulf of Mexico, we do it daily, regardless of hours flown. The salt in the air will quickly degrade the engine if it isn't washed out regularly. Our engines, washed daily, regularly make TBO. Without the daily wash, they won't come close. I don't have exact figures, but I suspect we get an extra 20% or more, but that would be less in places that don't have the salt in the air. Air pollution, though, can be just as bad if not worse. See the manufacturer's recommendations, as I said above.