Alcohol Shot Drinks

What is the time to hit a shot glass down once you drink alcohol? Luck, maybe, References?
I have seen this bars in a number of times the drinker slams the glass on the bar. Sometimes if is up, sometimes down, but with authority. The funny thing is that it is not the same class of persons, or from the same type of drink. A GF said it was for luck, but can not find anything on it.
It's stupid and no one knows why. All these correct answers are possible, but we have not definitively prove a source for one another, so choose one. I first saw in a Western movie, but to see it rarly in a bar.
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