It’s possible there is someone or a video who can show you though I came up with it myself in leu of spinning it so I don’t personally have any sources.

I can give you a written explanation though!

Step one: Hold the Pen as you normally would

Step two: Get the rubber pen grip (or associated area, roughly 1/3 the length from the tip) onto the knuckle at your finger tip or just in front of it (towards the tip of your finger). This should have your finger in a C position which should put you pretty close to a flick position.

Step 3: Put your thumb in front of the tip of your finger and apply counter pressure between your fingers (ready to flick)

Step 4: Release your restrained middle finger causing your finger to flick and sending the pen up in a flipping motion.

Notable: If the pen flys forward try adjusting how forward or backwards the pen sits on your knuckle (I.E. 1/3, 1/4 etc.). Same if the flip is too speedy or slow.

Adjusting your wrist or moving your hand up to start with some momentum can be useful when learning.

Consistency is hard but you’ll get the feel in time