In the States we say Knock on wood - and you do. If there is no wood within reach you knock on your head, ha ha. There is a great scene in Casablanca where Sam plays Knock on Wood (Now who's happy? We're all happy! and so on) so it predates that movie... that is all I know about its history. But it reminds me of the African and Jewish(and other cultures') "superstition" that if you say things are going well, they will immediately get worse. Where I grew up if someone said How are you (Ça va?) you'd say Barely OK (un peu), never Fine (oui, ça va). And you never commented on how nice the weather was. Same superstition but without the wood...