Its a very simple yes/no vote. A second ballot will either return continued strike action, or an end to the strikes. It really is that simple.
Strikes are costing guards money and making life difficult for passengers.
As a former trades union national committee member, not rail related, I've seen this sort of situation. A lot is bluff and counter bluff. In some ways it is simple, but it's not.
The union won't want another ballot in case it gives a majority to return to normal working. Neither does it want to see a weakening of the previous vote. On the other hand the employer is aware that a new vote might actually result in a stonger vote for continued action, and one validated by being up to date.
In some areas there will be strongly held pockets in favour of abandoning strikes. They may be counterbalanced, maybe overwhelmingly, by bigger groups eleswhere strongly in favour of carrying on. It just needs one manager somewhere to make a rash decision to unite the feelings of waverers in favour of striking. Of course a rash union action can send it the other way. Anything can happen in the time from declaring a new ballot to the date it is completed
I recall a long running dispute, banning overtime and working to rule. We tried to inconvenience customers as little as possible, and mostly did - except in a few highly militant areas. The union won, probably on the backs of those militants in a very few places. A few years later my job had been slowly deskilled, I was offered early retirement and my successor was paid less than 2/3 my salary. The job no longer exists - but we won! Or did we?
I'd love to get back into the details of the dispute, but at present both sides are losing.
Ultimately passengers are the ones justifying the jobs. Hack them off enough and they'll find other ways to travel, or stop travelling. Do we really need railways at all?