This Easter Egg involves turning Pikachu upside and when he turns right side up, his hat slings around. Oh yeah, this is only on the Pikachu wearing a Santa hat.
To get this to work, you must first go to settings and turn on battery saving mode.
Next Go to Pokemon, and select a Pikachu wearing a Santa Hat.
Turn your phone upside down, and it should go dark.
Turn it right side up again, and you should see Pikachu reappear and the Santa Hat tip sling around the screen.