This method of crochet chains at the bottom of your work let you create live loops that can be picked up afterward and be worked down, e.g. cuffs, lace borders etc.
Use a crochet hook and crochet sts upon a needle. Start with a slip knot.
After required sts are on the needle, don't pass the last loop onto the needle but chain a couple of extra sts and then pull loop a bit long through last stitch. This is the thread you will pull later to unzip the crochet sts.