I remember a discussion about chess visualization with my son back when he was applying to college, as chess was a key point in one of his essays.
He said that when he played a tough opponent he might have dozens of complete boards in his head. Some were different variations of the current board several moves ahead, others were him replaying past games that were similar, and others still were famous games he had studied to see if there were lessons there that could be applied here.
Interestingly, his heart rate during a difficult game is often 120-130, even while completely still. His resting heart rate is below 60.
