a provisional cast on is ANY cast on that is easy to undo.
there are several cast ons that work.
I like the EZ (elizabeth zimmerman one), but there are 2 styles of crochet provisional cast ons, and truth be told, any cast on will work
take some waste yarn close in weight to your yarn (doesn't have to be an exact match..) I like to use sugar 'n cream for medium weight, and crochet cotton for light weight)
COTTON or any no linty yarn will do.. (cheap yarn is better in general)
Cast on. (what ever number of stitches are required) USE YOUR FAVORITE CAST ON!
then work 3 rows --(or more if you are afraid! i like stocking knit, but you could work 3 rows of knits (garter))
cut waste, start knitting with Real Yarn.
to undo, take a small sharp scissors, and cut off cast on. [COLOR=Red]Cut as close to cast on edge as possible. [/COLOR]
then undo row1 of waste yarn.(it's likely to be in several pieces), then undo row 2--finally undo row 3 and pick up stitches.
Other provisional methods, waste less yarn. But 1 or 2 balls of Sugar 'n Cream (on sale, $1 each) will last you the better part of live time if only used for waste cast ons.
Eventually you can learn less wasteful methods--and really that is the main difference between the various methods.
EZ's style uses less waste yarn than the crochet style, and the method outline above waste the most! [COLOR=Red]but in the end, with all the different methods of provisional cast on, you unravel an edge, and pick up live stitches... [/COLOR]
Why make your self crazy and fuss over it? waste a few yards of cheap cotton. and later cut use a scissors to undo the cast on!
(me? i am crazy and know way to many ways to cast on.. including 4 of 5 provisional styles!)