Article XII, Section 2 of the IBT Constitution specifies what happens if there aren't enough votes...If enough of the membership doesn't vote the contract down it will be "imposed", so to speak, even if the majority of those who voted did vote "no".
Article XII, Section 6 was abused in the last contract to impose the supplementals that were repeatedly voted down by amending the language. It was claimed by Hoffa and his leadership that the holdout locals were voting down their supplementals because of language in the NMA which had already been voted in, and this was used to impose the supplementals.