Pressing one's thumbs with the rest of the fingers is the usual way to hope for luck in germany too :)
<img src="http://www.mediafire.com/imgbnc.php/e7df0cbf339667111d4cba5b301cdec62b9b40c346f46e3389db15733e76a1e32g.jpg" border="0" />
The roots are in the ancient german tribes. Thumbs were the most important fingers, so hiding them behind the others protects them. And, the thumb as the most important finger also able to do bad things, hidden in the hand proved that it would do no harm.