I'm quite conviced that L. da Vinci drew this grip somwhere, but I can(t put my hand on a precise ref. Damn.
It's the first time I hear someone is actualy using it.
A very intresting fact about this grip is that, if I'm not mistaken, for a given throw (let's say sidearm, palm forward,) your cords are switched (retention up, release down, right hand throwing), and your stone will spiral counterclockwise. therefore, a low density projectile (like a tennis ball) should curve to the left
Please keep in mind tht's just theory, I've never actually practised this.
EDIT: Here's a low quality version of Da Vinci's drawing. barely usefull but still...
EDIT AGAIN: This post might contain false infos as the grip depicted is not what I thought. I posted without a good verification. I will try not to do it again. Naughty boy.