 Your breathing is shallower, so your sound goes to the top side of the sound, and you sound a little shrill and a little strained. Your shoulders go tight, and therefore then everything's a little more pinched, not as open and resonant. These signs are all too familiar to musicians and athletes, who can often recognize when others are about to choke, even before it happens. One example was Patrick Chan in Vancouver. Representing Canada, Patrick Chan.