That is basically steam cleaning, steam is an extremely potent cleaner and will blast away built up solid soot too, but you have to be extremely careful to not let the engine gulp water as it can hydrolock it and cause a rod to bend.
It used to be pretty normal part of service till the 1960s when engines were quite rough and poorly made. Nowadays you never need it specially for a japanese origin engine (they are very well designed) as long as you are running it with the correct fuel and lube and drive it properly.