The slip stitch is useful when a crochet stitch is needed that doesn't add any height.
The slip stitch can be used to join the last stitch made to another part of a row or round.
2. Insert the crochet hook into the next stitch.
3. Wrap the yarn or thread over the hook.
4. Pull the yarn or thread through both the stitch to be worked and the loop already on the hook, all at the same time.
5. One slip stitch has been formed.