The principle can be concisely described as: A spectator is told to write a word or question on a slip of paper or card. On this piece of paper are instructions about what information he is to write; e.g., phone number, address, birth date, a friend’s name, a color, a city he has visited. The spectator doesn't question or mention this, and the rest of the audience isn’t aware of these written instructions. This principle was explained by Paul Kara in 1926, as “Karaism Effect #9: Mental Census” in his manuscript Karaism. This is reprinted in The Collected Mental Secrets of C. A. George Newmann, edited by Leo Behnke, 1990, p. 98.
Nineteen years later, James G. Thompson, Jr. used the idea in "Strongest Thought", marketed in 1945.
A more advanced version was devised in the early 1970s by John Pomeroy. In Pomeroy’s version, each slip secretly requests a different bit of information than the others, along with one piece of information openly asked from all the participants. This leaves not only the audience puzzled but the persons who filled out the slips, since their slip did not ask for the information being revealed to others. John Pomeroy’s method is explained in his book Mentology, 1973, p. 16. Further versions of the concept have been developed since.