There is nothing to understand. English is English. Italian is Italian. Where is the problem?

Every language has its own idioms and expressions that do not translate word-for-word from one language to another. Sometimes, they do, but most often, they do not. By the way, it is "knock *on* wood," (not "knock wood"). The French, for example, say, "touch some wood" (toucher du bois), which, while using the same word "wood" as in English, sounds vaguely naughty to someone who speaks English if translated directly. This is part of the fun of learning languages. If you could translate word-for-word from one language to another, we wouldn't need human translators and machines could simply substitute one word for the foreign word equivalent and give us a perfect translation... Anyone who has ever used google-translate knows this is not true!

In fact it is 'knock ON wood' and yes it seems this kind of superstitious behaviour - averting bad luck - occurs in all cultures and languages I guess. There is no 'why' as to the pattern or behaviour - 'basta' it is just so. For you Iron, for English speakers wood. Black cats, ladders ...