I hope this explains a slip stitch a little. It is from readers digest all about sewing.
Slipstitch
This is an almost invisible stitch formed by slipping the thread under a fold of fabric. It can be used to join two folded edges, or one folded edge to a flat surface.
Even Slipstitch is used to join two folded edges. It is a fast and easy way to mend a seam from the right side, especially one that would be difficult to reach from the inside.
Work from right to left. Fasten thread and bring needle and thread out through one folded edge. For the first and each succeeding stitch, slip needle through fold of opposite edge for about 1/4 inch; bring needle out and draw the thread through. Continue to slip the needle and thread through the opposing folded edges.