A definition of the Flex and DP offered by Glenn Colburn, NH ASA Umpire In Chief
About the Twins
You can't have one without the other
They are like "twins" and they are special. They have very special privileges but not without restrictions.
If the twins are in a game they must be in a starting line-up.
If not in starting line-up then you will never have twins in the game.
Always check your line-up. If 10, determine which is the hitter.
Each twin has their own position in the line-up.
The DP must play offense to be considered in the game. The DP has not left the game until she leaves her offensive position.
The DP can never play just defense.
The FLEX must play defense to be considered in the game. The FLEX has not left the game until she leaves her defensive position.
The FLEX can never play just offense.
Each twin can leave her position in the line-up and return to that same position one time during the game. (Re-entry).
Each twin may use the other twins spot in the line-up but not at the same time.
Each twin play both offense and defense but they both can't play both at the same time.
The DP is the dominant twin... she was born first. She has more privileges.
The DP can play defense for any of the other nine players in the line-up. (So the twins could be on defense at the same time.)
The FLEX can only play offense for the DP. (So the twins cannot be on offense at the same time.)