I often hear baseball players and golfers talk about focusing on the left or right arm in execution of the swing. For example, you should focus on the lead arm (left for a right-handed player) during the forward swing. However, the arms work together for one purpose.
It is impossible to separate the action of the left arm from the right arm when holding a bat or a golf club. Movement of one arm will automatically affect movement of the other arm and the batting implement.
You cannot focus on each arm independently since both hands are in relatively strong contact with the club. For effectiveness and efficiency of movement both arms must work in unison. They can work independently only when not holding an implement. Ait’st other times the arms work together.
If you begin using (emphasizing) one arm more strongly than another arm, you will create disruptions in your swing. But when the arms work together they can create and maintain a specific action to produce enhanced movement.
In general, it is more effective to think in terms of using both arms simultaneously when independent arm action is called for. Keep in mind that shoulder rotation usually gets the arms in motion to overcome momentum before the arms begin their separate (independent) action contributing to the buildup of force in the sequential kinetic chain.
If you are a golfer see Explosive Golf for more information on the role of the arms in execution of the swing. If you are a baseball player see Build a Better Athlete for more information on the arm action.