Abarim Publications' online Biblical Hebrew Dictionary
The verb זחח (zahah) occurs only twice in the Bible, and that in only one context. In Exodus 28:28, YHWH instructs Moses to attach the breastplate to the priestly ephod by means of a blue cord, in order to prevent it from זחח (zahah). And in Exodus 39:21 the making of the ephod is described using the same wording. The blue cord was obviously supposed to keep the breastplate in place; to prevent it from falling off or from swaying away from the priest's chest when he moved around.