I'm not sure exactly how it works - but the thrust of the trick is to get the person to choose the letter 'D' - someone else can hopefully refresh us on how this is done.
You then ask the person to choose a country that begins with the letter chosen - being D, 99% of people say 'Denmark' because that's what comes to mind, and that is the country you have written down in advance.
it is repeted but the trick is done like this.....think of a number between 1-9....times it by 9....now the digits that you have,add them together 'E.G'..18 1+8=9.....take away 5 and your left with 4....now wateva number they hav must be converted to numbers...A=1 B=2 etc etc.....They always end up with 4 and 4 is D in the alphabet then u tell them to think of a country beginning with that letter.....hope this helps
Denmark is the obvious one as people tend to think of that before they think of the less familiar Dominican Republic or Djibouti (the only countries in the world beginning with D).
Similarly if you asked someone to name an animal beginning with E, most would say 'elephant'.