If you want to sew details on a face, e.g. the whiskers of an animal, this stitch can be used.
Fig 1: Bring out the thread through A and take it in through B. Take the needle backwards and bring the thread out through C. Make sure the point C lies over the stitch A-B.
Fig 2: You need to note that the point C lies about half way through A and B. Also note that C lies on top of the stitch A-B. So, all the subsequent stitch points will lie on top their previous stitch.