Restringing is the process of tightening or replacing the elastic cord holding a doll together. Asian ball-jointed dolls are strung through with elastic cording. In some cases, this elastic may be too loose or too tight, as evidenced by the floppiness of the joints. At that point, often owners choose to restring their dolls.
Tightening the elastic is usually a matter of taking off the headcap of the doll and pulling the elastic taut, and putting a new knot in the cord.
Complete replacement of the cord requires the owner to fully take the doll apart, pull out the old elastic cord, and replace it with new.
There are a variety of tutorials on how to accomplish this, as restringing becomes more complicated the more joints and modifications are added to the basic jointing system.
Suggested Elastic Thickness for various sized dolls (this is just a guide, actual preferences may vary):
2mm Diameter - MSD & Smaller
3.5mm Diameter - MSD & SD
4mm Diameter - 60cm
5mm Diameter - 60cm & Bigger