What happens?
When the members of a union reject a tentative agreement (TA), then the union and the employer go back and try again.  Its a fresh start.  The union can demand more than its previous bargaining positions, like more money or less testing or more prep time and it is not considered “regressive bargaining”.  Because the members have spoken strongly then it is unlikely that they will get a worse deal.

It may make the leadership look weak, but it makes the union look stronger.

It takes a majority of the members who actually vote to approve or reject a TA, so its important to vote.  In UESF the lack of participation in most elections or votes is a powerful vote by itself.  That’s why a handful of people in UESF make all the decisions and most UESF members don’t know that there are decisions to make, that they are being made, or that they have a choice.  They certainly don’t feel powerful or know that unions are powerful tools, inherited from past generations who fought very hard to get them.

Bargaining teams (BT) work for the members of the union.  They represent the membership in the negotiations, and then bring their TA back and offer it, and request the members approval.  That’s why its “tentative”, and why there is a ratification vote.  The members decide.