In English people cross their fingers to show their support/wish good luck on something. In Poland however, we hold (more like squeeze) our thumbs to convey a similar sentiment.
Apparently, this custom comes from the ancient times, where warriors would hold their thumbs while praying for good luck in battle, as without them, they wouldn't be able to hold their swords, and therefore lose.
What is the equivalent in your language? Why do you use this particular phrase?