When you think of it, Americans' use of "the finger" to deeply degrade someone is a pretty small, meek hand gesture, considering the hefty insult it's supposed to convey. The European forearm jerk, in contrast, is a much meatier gesture.
First you take your right hand and make a tight fist. Then you jerk your right forearm up as you slap the bicep forcefully with your left hand. Southern European males, including those in France, who call it the bras d'honneur, use the forearm jerk as a crude, phallic way to flip someone the bird. It can also indicate sentiments such as, "I'm better than you are," "Get lost, loser!" or "Up yours, buddy!" In Brazil, the gesture is known as the banana, although its meaning is the same. Men in Britain and Germany sometimes make the forearm jerk as a rude way of indicating they're lusting after a particular woman, although they'd never make the gesture at the woman [sources: Kruschewsky, Steves].