Touch wood is a humorous and superstitious expression that is supposed to prevent bad luck. Very often people actually touch wood when they say it. But there is enough evidence in the Bank of English to show that people do not touch wood while saying it. Perhaps that is because they cannot reach any wood while they are saying touch wood, or because saying the phrase is an adequate replacement for doing the action. In the United States, the expression touch wood is rarely used, instead, the expression knock on wood is more commonly stated. For the purpose of this study, touch wood will remain the key phrase as it is listed in the Bank of English.

Some peculiar characteristics of touch wood
Touch wood sometimes occurs with other expressions that also indicate an orientation towards "luck". For example, 1) What would happen if they found out about us? Gosh, don't even talk about it. Touch wood quick. He reached out for the bedside table. Corpus: brbooks/04. Text: B0000000129.
2) Well, I said, as long as it costs less than three quid [British pounds] a bottle I don't have a problem (Janet Lee, Tesco's senior wine buyer who is behind the idea, didn't see me frantically touching wood with my fingers crossed). Corpus: guard/13. Text: N7000950513.
3) Just keep my fingers crossed and touch wood you know everything goes all right with this FX on Monday.
Corpus: brspok/07. Text: S9000000460.

When to use 'touch wood'...
It seems that there is a popular idea in some parts of the West that if people say they have some good fortune then it will disappear. To maintain good luck, people say touch wood while actually touching a piece of wood or just say touch wood. Here are some examples:
Note that often speakers make a reference to the temporary nature of the good luck by using a time reference such as "so far". Unfortunately, the Bank of English does not have comparable data for American English.

1. He said: "I've seen accidents. Touch wood, I've still got all my fingers." Corpus: today/06. Text: N6000940730.
2. But so far touch wood they haven't gone wrong. Corpus: brspok/07. Text: S9000000576.
3. "Anyone else in the family with tummy bug or anything?" "No one else. No. Touch wood everyone seems to be okay at the moment." Corpus: brspok/07. Text: S9000000811.

In each of the examples above the speaker describes a piece of good fortune. In each case, the good fortune of this kind may disappear. For example, people who are healthy may become ill.

Occasionally, there are cases when people just pray for good luck by saying or doing so. For example, 1. So I'm just hoping like touch wood that I don't get caught. Corpus: brspok/07. Text: S9000000485.
2. Do you think that that kind of situation's likely to happen again? Or, er ... touching wood, I hope not. Corpus: brspok/07. Text: S9000000973.
How to use "touch wood" in a clause 1. It may be used at the beginning of a clause. For example, 1) And touch wood no one's died. Corpus: brspok/07. Text: S9000001221
2) Oh I'd sit in my class and I'd bend under the table and take my socks and my shoes off and I'd be digging away at my toes. Oh it was awful I hated it. But touch wood I've never had it since. Corpus: brspok/07. Text: S9000001181.
2. It may be used at the end of a clause. For example, 1) I've been lucky to avoid serious injury (touch wood). Corpus: brmags/10. Text: N0000000651.
2) But you know here we don't have that problem touch wood. Corpus: brspok/07. Text: S9000000754.
3. It may also be used in the middle of a clause. For example, 1) If he had had a good fit which touch wood he hasn't had since last July, Š Corpus: brspok/07. Text: S9000001276.
2) Now if the club were again, touch wood, to be successful tomorrow nightŠ Corpus: brspok/07. Text: S9000001118.
4. The Cobuild data above show that the present indefinite tense is the major tense that touch wood uses. Occasionally it is in the present or past continuous tense.

There is only one case for the use of touch wood in the past tense, which is used to describe the action of touch wood in the past.

As can be seen from the examples above, touch wood is usually used in the imperative, though other forms are not excluded.

After an investigation of these three non-verbal behaviours in the Bank of English we may have a better idea of the function and use of shrug (January/February 1999), hold people's gaze (March/April 1999) and touch wood. But notice should be given to the fact that there may be variant expressions to mean the same thing.

As pointed out in an analysis of shrug; shrug one's shoulders has the exact meaning as shrug. Several other words have been found to replace hold in an analysis of hold + one's gaze such as meet, and return. However, no equivalent has been found to touch wood even though keep one's fingers crossed shares some function of touch wood. People from different cultures had better compare these non-verbal signifiers with those in their native languages before actually using them.

Of course, it is sometimes impossible to find the correspondent in the system of one's own non-verbal language as is the case in verbal language. Using a foreign language always involves the use of non-verbal language such as gestures, and eye contact. English learners do not need to shrug to native English speakers in facial interaction at the first stage. But it is the author's belief that it is a sign of mature acquisition of English if someone is able to communicate smoothly both verbally and non-verbally with native English speakers.