I’d never felt comfortable trying a the psychological stop force until watching disk 3 of Dani DaOrtiz’s DVD set UTOPIA. He breaks down the timing of the move and it explains it better than anyone I’ve read/seen before. I starting doing Dani’s “Numerical Match” routine and you really do start to get a feel for when the spectators will say stop, – it’s wierd. When done right I think the performer actually slows the pace slightly near the end to almost urge them to stop then. Of course what’s so great is that Dani also provides complete fool proof outs if you miss so you really can’t go wrong.
I think many card tricks can be made so much more miraculous if we try adding in some psychological forces. The complete belief on the part of the spectator that they had a complete free choice of when to stop is powerful stuff.