Jamieson, Fausset and Brown on Jeremiah 34:18

18. passed between the parts thereof--The contracting parties in the "covenant" (not here the law in general, but their covenant made before God in His house to emancipate their slaves, Jr 34:8, 9) passed through the parts of the animal cut in two, implying that they prayed so to be cut in sunder (Mt 24:51; Greek, "cut in two") if they should break the covenant (Ge 15:10, 17).