There are actually two laws.
Law 1. As the intensity of motivation increases, the quality of activity changes along a bell-shaped curve: first it increases, then, after passing through the point of the highest success rates, it gradually decreases.
The level of motivation at which the activity is performed as successfully as possible is called the optimum motivation.
Law 2. Things of medium complexity are best done with an average level of motivation. The more difficult the activity performed for a person, the lower the level of motivation is optimal for it.
Details
The Yerkes-Dodson law in psychology is the dependence of the best results on the average intensity of motivation. There is a certain limit beyond which a further increase in motivation leads to worse results.
As early as 1908, scientists established that in order to teach animals to go through a maze, the most favorable is the average intensity of motivation (it was set by the intensity of electric shocks).
It is known that in order to carry out activities, sufficient motivation is necessary. However, if the motivation is too strong, the level of activity and tension increases, as a result of which certain disorders occur in activity (and behavior), that is, work efficiency deteriorates. In this case, a high level of motivation causes undesirable emotional reactions (tension, excitement, stress, etc.), which leads to a deterioration in performance.
It has been experimentally established that there is a certain optimum (optimal level) of motivation at which the activity is performed best (for a given person, in a specific situation).
For example: the level of motivation, which can be conditionally estimated at seven points, will be the most favorable. The subsequent increase in motivation (up to 10 or more points) will lead not to an improvement, but to a deterioration in performance. That is, a very high level of motivation is not always the best.
Criticism
The experiment does not speak about motivation in general, but mostly negative motivation (electric shocks). If you motivate positively, then there will be no fear and stress, and an increase in motivation will not lead to a decrease in performance. Learn to live with positive motivation!