This is a typical Western unsupported link stitch. To start off this type of sewing the first two gatherings are sewn to each other first, creating a link between the two at each station. Each following gathering is then secured to the previous one by passing the thread through the loop of the previous attachment it to create the alternating chain design.
Sewing on recessed or sawn-in cords - it was also used as a time-saving device. A groove is sawn into the back (spine folds) of the text block where sewing stations will be. Cords are placed in the grooves & sewn over all along. Instead of the needle going out each hole, around the cord, & back in, the needle passes directly behind the cord. Cord is visible on inside of the gathering & the needle does not go outside the holes at all, thus making it faster to sew.
This is another style of link stitch called French sewing. This style of sewing can be sewn over tapes (flat sewing supports) or without (as pictured here). As the thread is linked through the sewing of the section before, the stability of the structure depends on how many sewing stations there are and how far apart the sewing holes are. This version is unstable and would need more stations with the holes closer together to improve the stability.