For those who struggle keeping the arms from dominating the swing, swinging a broom will quickly ingrain the proper rotary swinging motion required for the one plane swing. Hold an ordinary house broom in front of your body with arms extended. The wide profile of the broom should be perpendicular to the ground.
Swing the broom back behind you and then swing back through, noting the massive amount of resistance that the broom provides. If you swing the broom through impact aggressively with the arms, you should note two things.
One, because of the resistance, it does not swing through very smoothly unless you allow the rotation of the body to gradually accelerate the broom. And two, if you try and swing the broom with arms more so than the body, it will make a “swooshing” sound twice, once around impact and again there after.
The desired affect is that the broom makes a gradual swooshing sound that reaches its peak well out in front of the body rather than near impact. This lets you know that you used your body to accelerate the broom and not the arms.
You should also note that using your body to accelerate the broom makes the swing feel much smoother, whereas it will feel somewhat jerky if you over use your arms.