וְיִרְכְּס֣וּ אֶת־הַ֠חֹשֶׁן מטבעתו [מִֽטַּבְּעֹתָ֞יו] אֶל־טַבְּעֹ֤ת הָאֵפֹד֙ בִּפְתִ֣יל תְּכֵ֔לֶת לִֽהְי֖וֹת עַל־חֵ֣שֶׁב הָאֵפ֑וֹד וְלֹֽא־יִזַּ֣ח הַחֹ֔שֶׁן מֵעַ֖ל הָאֵפֽוֹד׃
And they shall bind the breastplate by the rings thereof unto the rings of the ephod with a thread of blue, that it may be upon the skilfully woven band of the ephod, and that the breastplate be not loosed from the ephod.
Read kabbalistic (mystical) interpretation of Exodus 28:28: and classical mystical sources.