Finger Trap Party Trick

Sometimes it's the simplest ideas which can prove so infuriating! The Finger Trap Party Trick is a traditional Chinese puzzle that goes back a very long way. Simply push one finger in one end, another finger in the other end, and then try to pull them out. It's impossible! The harder you pull, the more the Finger Trap Party Trick grips your finger and won't let go! The secret's simple, but can you work it out?