Maybe you only have the one hand. Maybe you want to show off. Maybe you're just too cool to deal cards with both hands. Whatever the reason, this is a skill that will elevate your status from card player to master card manipulator. Okay, maybe not. Still, it's fun to do.
(See the youtube video clip)
Step 1: Mechanic's Grip
Hold the deck of cards in your hand as if to deal. (Note: instructions are left-handed, but if you want to do this with your right hand, just reverse the directions.)
The thumb rests across or near the upper left corner of the deck. The index finger supports the cards by the top edge of the deck, and the remaining three fingers grip the deck on the right edge.
Experiment with the vertical positioning of the deck. Initially, I tried holding the cards closer to the top edge, so that the lower right corner of the deck stuck out of my hand. I later found that I was able to get more leverage on the cards (later steps) when I held it in a normal mechanic's grip (dealing grip).
Step 2: Grip the Card Between Your Fingers.
With your thumb, push off the top card as if to deal, then stick your ring ("fourth") finger under that card. Pull back the thumb, bringing back the card to the squared position.
At the same time, bring the middle finger down on top of the card, pressing down on it next to the ring finger. Lift the thumb off the card, and press down on the card with your middle finger. The top right corner of the card should be pressing against the rest of the deck, causing the card to bend slightly.
Step 3: Flick!
Gently extend the ring finger under the card. The middle finger remains mostly stationary, but bears down slightly to maintain the tension on the card. The top right corner of the card bends under the pressure from the middle finger. As the ring finger extends, the card slides along the top of the deck until it slips off the deck. The card should fly off, spinning as it does so.
Step 4: Final Notes
With practice you can improve your precision, and maybe use this as a way to deal cards at a poker night. Just don't blame me if you get accused of cheating.
I have never seen this sleight anywhere else, so I'm going to claim creatorship of it since I did think it up myself. Please let me know if you know of a previously published version of this.