There are two distinct types of psychological force. One is akin to "Think of a number between one and ten" and guessing seven because that's the most common answer. Banchek's Psychological Subtleties deals extensively with that kind of thing, showing what these predictable answers are and how to construct your question to more reliably elicit those responses. The other type is where you take something that is maybe not so readily predicted, and suggest it to your participant through your words and gestures. This is Docc Hilford, Kenton Knepper, Derren Brown and Luke Jermay territory, and, to my mind, is a bit more of an exciting area.