Alternative method of seaming

Some years ago a friend of mine was taught an alternative system of seaming up a garment. This method avoided a thick seam which results from the traditional mattress seam or the backstitch seam. Admittedly it is more noticeable, but still very nice.

Alas, she has lost the instructions on how to do this. It does involve slipping the first stitch of every row thus providing a loop for the stitches in the end.

Does anyone know of this method and can you describe if to me please. We tried to teach me this technique when I was
visiting her in England recently, but I am still confused