So what is a play psych? In spite of the almost endless card combinations in a bridge hand there are only a few basic ways that hands play out. One boring variation occurs mostly in NT hands. Declarer is trying to set up long suit tricks in his suits and opponents are doing the same in their suits. The effect is something like a drag race. The green light goes on, both sides mash the pedal to the metal, and whoever reaches the finish line first wins.
Another style is more like a sword fight, a lot of thrust and parry. Declarer is trying to do things to further his cause and the opponents are trying to stop him. If declarer is trying to set up a suit the opponents may hold up their high cards or attack his entries to the suit. If he's trying to ruff a suit in the dummy they may decide to lead trump. That's the kind of hand that lends itself to play psychs. Declarer can simulate taking a standard line of play and then make use of the defender's counter-play. Here's an example.
West leads the heart Queen. We win the King. Now what?