Extend your arms in front of you, and put your palms together. Slide your right hand over your left, so the right palm is touching the back of the left hand. Now flip your left hand so your palms are touching again. Interlock your fingers, and bring your hands up toward your chin. Have a friend point to, but not touch, one of your interlocked fingers. Try to wiggle that finger only... you might find you wiggle the wrong finger!
Your brain, eyes, skin, and muscles all have to work together to wiggle a certain finger. When your hands are crossed this funny way, the fingers of your left hand are in a position usually occupied by your right hand, and vice versa. When your brain says, "Wiggle that finger over there on the right side", you are accustomed to wiggling a finger on your RIGHT hand. So that's the signal your brain gives to your muscles, and Whoops!, you wiggle the wrong finger by mistake. With practice, you can learn to wiggle the proper finger, even with your hands crossed.