Put on a long-sleeve sweatshirt or sweater with loose arms so the positioning of your hand and arm will not be noticeable as you perform this illusion.
Get on your knees and place your left hand down on the ground with your hand positioned so that the thumb is closest to your body and your elbow is facing outward.
Place your right hand down on the ground at the same time, but subtly turn your hand around to the right in the loose sweater before doing so. Your arm will be twisted now so that your elbow is facing inward, but both arms should be straight so you can't tell the difference between the two arms. The thumb of your right hand should still be closest to your body, so in the loose sweater, it isn't noticeable that you have your arm twisted.
Ask a friend to put his hands on the ground positioned the same as yours with both thumbs closest to his body--without him twisting his arm, of course.
Tell your friend to look at your hands and copy what you are about to do.
Turn your right hand counterclockwise slowly until you have twisted it back in a complete circle to the exact position as your left hand. This will appear that you have twisted your hand around 180 degrees in an unnatural way.
Watch as your friend freaks out and asks you how the heck you did it.