1. Lift your right foot off the floor and move it continuously in clockwise circles.
2. While continuing to rotate your right foot, raise your right hand and draw the number 6 in the air.
If you write the number six, your foot will change directions no matter how hard you try to resist.
Now, rotate your left foot clockwise while writing the number 6 in the air with your right hand. You should be able to do both simultaneously with little effort.
So, what gives?
It's probably the case that the neural pathways directing limb rotation on the same side of the body share a common brainstem/spinal pathway. Thus, writing the number 6 (a counterclockwise movement) while making a clockwise rotation of the same-side foot is impossible. The shared neural pathway can't carry conflicting instructions. But if you try to do this with right and left foot, or left hand and right foot, the impossible task becomes easy because you're using two different pathways to carry the opposing rotational signals.