You place a shot of your choosing in the middle of the table, and cover it completely with a ice bucket (or hat, anything that hides the glass from all angles). You bet your mark that you can drink the entire shot without touching the bucket, you won't lift or slide the bucket. Let the mark put their money on the table, look at the bucket, pick up the money and tell them the deed is done.
This is all in the acting. You have to convince your mark that you really have done it. They will be burning to know if you have actually done it. After all they can't see through the bucket, and would have to take your word for it without checking. This is where they make a mistake, and pick up the bucket. Once they do, quickly pick up the drink and down it. You made good of your bet to finish the drink without touching the bucket.