a quick google reveals.....

There has been much confusion on alt.fan.pratchett concerning what exactly constitutes the 'ginger beer trick', and which bodily orifices are involved. Terry says:

"To save debate running wild: I've heard this attributed to the Mexican police as a cheap way of getting a suspect to talk and which, happily, does not leave a mark. The carbonated beverage of choice was Coca-Cola. Hint: expanding bubbles, and the sensitivity of the sinuses.