JDinPA,
"Crawl" is just a sort of slang term for string-walking, as in, you kind of "crawl" your hand down the string.
The point of string-walking is to keep your anchor the same, but get the nock high enough that the tip of the arrow is directly on the spot you want to hit at a given yardage. You're basically using the tip of the arrow like a sight pin of sorts.
For example: without walking the string, my Warf is point-on at 35 yards. If I come to anchor and put the tip of the arrow on the bullseye at 35 yards, that's where the arrow should hit. But by "crawling" my hand 1 1/4" down the string and coming to the same anchor, I've gotten the nock closer to my eye. Now instead of being point-on at 35 tards, I'm point-on at 20 yards.
So for bowhunting, if a broadside deer is 20 yards away, I can come to anchor, put the tip of my arrow behind his shoulder and shoot. If he's a little closer than 20, I can just hold the tip of that arrow a little low (4" actually). If he's a hair past 20, I just hold a little high.
For a great how-to on string-walking, you may want to check out Masters of the Barebow, Volume 2. Ty Pelfrey has a great section in there where he really goes into a lot of good detail on how it's done.
Personally, I'm more of a regular gap shooter (instinctive on occasion too). But I used to string-walk competitively years ago and always had a itch to try making it work for bowhunting. So I figured I'd set this bow up for it and see if I could turn some of that white Velcro red.