It's maybe because I am not a native English speaker but I think Autoglym's instructions were quite easy to understand: "Autoglym Engine & Machine Cleaner is a ... ... cleaner for engines, components, wheels and machinery. Simply spray on, brush and hose off to rapidly dissolve oil, grease, grime and traffic film from all surfaces. "
But on the other hand, Zaino recommends not to hose down the engine bay: "I have heard different ways on cleaning the engine compartments, including hosing it down. I personally will never do that. Again, I am not saying my way is the best, but you can see from the pictures I get decent results."
So there you go, even the product vendors seem to disagree on the topic.