The definition of poise is no different on the playing fields as it is in other aspects of life. The dictionary definition of poise says it well "a state of balance." With athletes, I believe this relates not only to the mind but also to the body's state of balance.
The one area of hitting in the major leagues that I was most proud of was my two strike hitting. I was in that situation often as I took a lot of pitches because of a lack of confidence with swinging early in the count. Being a singles hitter, I had the ability to battle and make contact when I had a two strike count.