I'm not sure who you were asking to give more details, but I can tell you how it described it in the book (as soon as I find it).
Here's some of the passage about it:
"She pondered the idea. If I could get a second stone in the pocket on the downstroke after the first hurl, without stopping it, I could throw it on the next upstroke.... She began to develop a rhythm: throw the first stone, catch the sling as it comes down with the second stone ready, get it in the pocket while it is still moving, throw the second stone."
I don't know if that will help at all, but I hope so.