This action is an example of affect display or kinesics.
Step-by-step explanation:
Affect display is an example of non verbal communication in which someone does some gesture from their body or face that betrays their emotional state or what they are thinking.
It is evident in the passage that the batter is
- nervous
- eager to outplay the pitcher
The biting of the nail while watching the pitcher closely is a telltale sign of this.