It's exactly what it seems like it is. Pinch between the pads of the middle finger and the thumb in the center of the card. The pad of the index finger contacts the underside of the card slightly forward of the middle finger. At the same time you will do three things - pull the index finger straight back, push the middle finger straight forward (very slightly), and lift the thumb a fraction.
The backwards motion of the index, combined with the forward motion of the middle finger, provide the force for the spin. Lifting the thumb a fraction provides a pivot point.
You will drop and throw that card over and over. Be prepared. Practice over a surface where you can retrieve the card easily, or keep a whole deck near you and be prepared to pick them all up later.
Also, once you get better at it you can flatten out your hand more and it will balance and spin on the middle finger at the same time. Also, I've never gotten this good but I've seen others do it, you can eventually get to the point where you can properly balance it and keep it spinning by moving the middle finger in a circular motion (I think they do it in the Propaganda deck trailer)