Place the ink pen between your thumb and index finger, as if you were going to write with it. If the pen has a cap, you may do the trick with the cap on or off, whichever is the most comfortable for you.
Put your middle finger underneath the pen to support it.
Fling the pen forward using your wrist, while continuing to hold it lightly between your thumb and index finger. As you fling the pen forward, turn your hand until the palm is facing completely downward. The pen should continue its forward momentum until it is about 1/4-inch from your wrist and underneath your hand.
Slip your middle finger underneath the pen so that the pen is being held underneath by both your middle finger and thumb and on top by your index finger.
Turn your hand sideways and allow the momentum of the movement of the pen to allow it to sit between your middle and index fingers. The pen should be parallel to the floor at this point.
Push the pen forward until it comes to rest back in its starting point between your thumb and index finger, in the writing position.
Repeat this process again and again, as fast as you can, so that you get a continual movement of the pen through all of the various positions without it stopping. The longer you are able to keep up the continual movement of the pen through the correct positions, the more like a moving infinity symbol (or "figure eight") the pen will come to resemble.