An arms only swing is common amongst senior golfers, as the older we all get, the less flexibility we all have in our bodies and with reduced flexibility, it becomes more difficult for us to make a good backswing by rotating, or turning, our upper body.
An arms only swing is exactly that. It is a swing with very little rotational body movement that just really uses your arms to move the golf club.
As you move the golf club away from the ball, to the right (for right handed golfers), your arms will lift up to get the golf club to the top of your backswing and there will be a limited amount of upper body turn to the right. Your arms become the dominant element in your swing and as a result several issues occur.
The first issue is a loss of power. Think about how you can throw a ball. You could stand quite still with your body and just use your arm to propel the ball forward. Or you could draw your arm back and turn your body away from the direction of the throw, get your body weight on to your back foot and then rotate your body into the throwing action along with your arm. This throw will always achieve more distance as by turning your body and using the power that is generated from this turn, the ball will be propelled further. The same is true in your golf swing. If you are only using your arms to swing the golf club, you are only able to put the strength and speed of your arms into hitting the ball. If you learn to rotate your body more during your swing, you now have the power of your body and your arms to strike the ball with, helping you to produce more distance to your golf shots.
The second issue with an arms only swing is that you will experience a lot of inconsistency with striking the ball. If you only use your arms, you will lift the club up away from the ball in a very steep, high action on your backswing. As a result of this, you will swing the club back down towards the ball in a very vertical, steep action at the top of the ball. All of your energy will be moving downwards towards the ground, rather than towards the target and as a result, the club head will either hit the top of the golf ball or hit the ground, making it very difficult to strike the side of the golf ball repetitively.
The final issue with only using your arms to swing the golf club is with directional control of your shots and accuracy. As you swing the club away from the ball, because you have little rotational upper body movement, the club head stays relatively close to the target line. As a result, when you swing back down towards the ball, your club head will be pushed to the outside of the target line and you will then have to pull the club head back towards the ball to strike it and the club head will continue to the inside of the target line, producing an out to in swing path. This will now make it very difficult to hit shots straight at the target and you will tend to see your shots going left or curving left to right.
To improve your upper body rotation, try the following drills. First of all, step off the range mat and set up so that the mat is directly next to the club head, providing resistance to you moving it away on your backswing. Begin your backswing and notice which part of you instigates the movement. It needs to be your left shoulder moving the club away (for right handed golfers), not your hands.
Secondly, to get your upper body rotating in your swing, place your golf bag in line with yourself but to your right, about 1 stride away from you. Set up as though you are going to play a shot, but without your club. Now imagine your friend is on the other side of the golf bag, holding their left hand out to shake yours. Without moving your feet, reach over the bag with your left hand to shake theirs. You have now rotated your upper body as you need to in your backswing. Work on this to get the feeling, then begin to hit balls, copying the movement.
Working on improving your upper body rotation will increase your power and distance, improve your accuracy and will deliver much more consistency to your golf game.