When hitchhiking in the western world (e.g. North America, Europe), one signals this by standing beside the road and with an outstretched arm, fingers curled inwards towards the palm, and the thumb sticking up at a right angle, or as close to it as possible.
Hitchhiker's thumb is a term for hyperextension of the thumb. This shows up in biology classes because in humans it is an autosomal, recessive trait. People who have two recessive genes for this trait can bend the distal joint of their thumb back so that it forms an angle of almost 45 degrees. (You then can run Hardy-Weinbergs on the class. Great fun is had by all.)
Other fun things along this line are Mid-digital hair, Tongue Folding, Tongue Rolling, Interlocking Fingers and Thumbs, PTC Tasting, and Sodium Benzoate Tasting.