It's the classic thumb trick, you make it seem like your thumb is splitting: One hand horizontal, palm facing towards you. All fingers close together, the nail of the thumb hidden behind the hand. The other hand vertical, behind the other hand and palm facing away. Thumb is also bent, but now your index covers the knuckle of the thumb, leaving exposed the thumb nail. The idea is that you put both thumbs together by the knuckle to make it seem like one finger, and then you move the hand behind to make it seem like you're splitting the thumb in two.