You can even throw katana with this style, Ralph shown it on the DVD. Can you imagine how much kinetic energy that would have?? Even pieces of flat iron, tanto pointed and sharpened, can stick deep...
I cannot take pictures now (my Mavica seems dead), but the grip is like that: take a knife(or marker or whatever thick enough), lay your index finger along the spine and your thumb on the side of the handle. Then you must do something so the knife won't fall out of your hand, and that's gripping it on the underside with your middle, index and pinky finger. Congrats, unless you were lost somewhere through this description, you should hold the knife in something closely resembling the overhand grip! For underhand grip, turn the blade 90 degrees sideways(on long axis, of course), so the index finger is on the flat of the blade and the edge is pointing away from thumb side of your hand. That's it. I will try to revive my camera and take some pictures.
CanDo: No, as atlatl is powering the dart from the very end. The index finger is there mainly as a guide - the overhand throw is a bit similar to pointing your finger on somebody using an overhand motion... try to remember if you have a knife in your hand if you like fancy gesticulation - otherwise you may easily impale someone.
loh_kah_hoe: Yes, you can throw anything long and heavy enough, the sidearm throw is best for that. Big nails with heads cut off are pretty good for it - they glide smoothly from your hand.
Taiki: Well, PDF isn't going to be made - at least not by me. Ralph Thorn was kind enough to send me the book and DVD for free(I promised him that I will review it for a Czech e-zine about unconventional warriorship and I kept that promise), so I would feel ungrateful if I would use that favour for making a PDF out of it. I have nothing against PDF'ing books from companies like Paladin Press and such, but this is a different thing. Sorry.