How many cords is the Pasuk referring to?
Rashi (in Pikudei): There must have been at least two cords to tie the Choshen to the Eifod via the rings on the two sides. 1
See Rashi in Pikudei, 39:31.
What is the meaning of the word "Lo Yizach"?
Rashi (citing Dunash ben Lebret): It is an Arabic word that means 'shall not move away (detach').
Ramban: Based on the fact that a "Zayin' and a "Samech' are interchangeable, 1 it means 'demolish (break-up').
Yoma, 73b: Anyone who moves the Choshen from the Eifod is subject to Malkos.
Why is it forbidden to move the Choshen from the Eifod?