ever wonder why people do a "salute" or hit someones head when a beetle passes?

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a volkswagen beetle that is. yesterday, on my way to church, i was looking at this couple. when a yellow volkswagen beetle passed, the girl hit her bfs head. and they laughed. i don't get the gist. and then my friend also does a salute whenever she sees a volkswagen beetle on the road. can someone explain or help why they do this? it's a little funny actually. LOL