In the article Get in the Zone: The Psychology of Video Game Design
What does Millar mean when he refers to players being in a “state of flow?”

 involved in an activity for its own sake. The ego falls away. or the mental state in which a person performing some activity is fully immersed in a feeling of energized focus, full involvement, and enjoyment in the process of the activity. Wikipedia

Explanation: According to positive psychologist Mihály Csíkszentmihályi, what you are experiencing at that moment is known as flow, a state of complete immersion in an activity. He describes the mental state of flow as "being completely involved in an activity for its own sake. The ego falls away.

major league baseball

explanation:

jack roosevelt robinson broke the color barrier in major league baseball.as he was the first african american to play in the major leagues, jackie robinson became the target of racial abuse. jackie robinson is referring to the historic event, harlem riot.