I'm not exactly sure what the classic force is (I don't do very much card magic at all) but if it is what I think it is, then it works 9/10 with me. When I perform a card effect, I treat shuffling, cutting, selecting a card, etc. as simply
going through the motions, i.e just something that has to be done before the effect proper, begins. Spectators assume that the deck was fairly shuffled and there is no way I could have known, or had any control over which card was selected.
The only time it doesn't work is when a spectator takes the card selection
too seriously. However, this usually only occurs when a spectator believes that the effect will simply be, he picks a random card and I 'magically' tell him what it is. If it is made clear before the effect that the trick is going to be more than that, then the shuffle, cut, select, becomes a mere preliminary to something about to happen.
Of course if the classic force, is not what I think it is, then ignore my answer.
