Janet emailed in this question (thanks, Janet!)
Me, personally, well, to be quite honest, I don't spin from the fold all that often. But Abby has done a terrific example on this in her "how to spin a batt" pictorial on flickr (remember mine? LOL great minds think alike! Mine features a V join, not spinning from the fold.)
The basic idea is to break your batt, roving, sliver, or top into staple-length long pieces. Staple-length is the length of individual fibers in the batt/roving/etc.
Spinning from the fold is done to help put air back into worsted-type preparations like top or even carded batts, which tend to have the fibers more aligned than not. It can be very useful with long staple fibers such as silk and longwools, since it cuts the effective "staple length" for spinning in half. (So don't try this with cotton, ha ha).
Since spinning from the fold is to re-introduce air, it is best when accompanied by long-draw or at least semi-long-draw style spinning: don't compress as you spin. If you must pinch, pinch lightly, and don't squeeze the length of new yarn, or you'll squeeze out all the air you just introduced.
So, once you have your staple-length-long segments (usually about finger-width, too), you fold one in half over the top of your index finger (you pick which hand, I use my left, and I'm a righty), and start drafting from the front middle, the top of the fold at the end of your index finger. The fiber is folded in half as it is drafting, and it pulls the fibers behind it out so they all stay folded in half -- once you get the hang of it!
When you are near the end of one segment, you add in the next.
Be sure to see Abby's pictorial. There are also some great instructions on spinning from the fold and a closeup on ispindle.com. This is on a spindle, wheel techniques are similar.
I hope to try this myself on one of Abby's batts soon. I've been spinning my first batt from her more worsted style, compressed and tight, for socks. But the second one, I want to try introducing a more woolen, airy aspect to it, and Janet asked, so I must revisit my spinning-from-the-fold skills (grin).
Questions? Comments? Post away, they are always welcome!