C. F. Keil and Franz Delitzsch on Exodus 28:27-28

Exo 28:27-28

Two golden rings were also to be put “upon the shoulder-pieces of the ephod underneath, toward the fore-part thereof, near the joining above the girdle of it,” and to fasten the choshen from its (lower) rings to the (lower) rings of the ephod with threads of hyacinth, that it might be over the girdle (above it), and not move away (יזּח Niphal of זחח, in Arabic removit), i.e., that it might keep its place above the girdle and against the ephod without shifting.