A tunnel stitch is used for an invisible seam, often in the context of protecting the thread from abrasion and in that way prolong the life of the object where it is employed. Especially on shoes a tunnel stitch was a common method to attach the sole to the shoe. The idea is that the stitch or thread only passes about half the thickness of the leather, sufacing again on the same side where it went in.
Preparing a sole for sewing on with a tunnel stitch: First soak the sole in warm water for a while (depending on the hardness of the material) to soften it. for example the sole leather I am using (4-5 mm thick) is very, very hard and I leave it 20-30 min in the water. After marking out the tunnels I pierce the leather with a slightly bent awl, as available from leather working supply shops. First a few millimeters from one side of the tunnel, then from the other side all the way through so that the tip of the alw re-emerges from the first hole. When doing so I bend the leather a bit against the curve of the tunnel to make it easier to get "around the curve."