I figured out his trick. So he asks Seungyoon start with any four numbers (in this case 9548). Seungyoon adds his passcode to it. Then he presses clear and gives the phone to the magician. The magician does a quick slight of hand. He presses enter again and seungyoon's passcode will appear on the calculator. The magician sees it, presses zero again and pretends to figure it out. It's easy actually.