Yerkes-Dodson Law of Arousal
The Yerkes-Dodson Law of Arousal, also known as Arousal Theory, states that an organism's performance can be improved if that organism is aroused in some manner. However, if the level of arousal increases too much, performance decreases. Of course, this level is different in everyone.
An example of this is an athlete who performs better under real game situation than he/she does during practice games. There is more arousal (stress, excitement) during the real games which increases their performance. But, if the pressure becomes too much, their performance can decrease (e.g., missing an easy shot with time running out and losing the game -- choking!).