I will refer to the convention by day number here, because while this is generally applicable, this year's Worldcon is one day earlier than the "traditional" dates and I sometimes fall into using the "traditional" days by mistake.
Day 1 = Wednesday, August 19
Day 2 = Thursday, August 20
Day 3 = Friday, August 21
Day 4 = Saturday, August 22
Day 5 = Sunday, August 23
Day 2 of Worldcon is the Preliminary Business Meeting. This meeting is the agenda-setting meeting, where we try to work out any conflicts in proposals (including amending new proposals or sending them to special committees with instructions to come up with something more workable for the following day) and establish debate-time limits for the Main Business Meeting. The PBM also receives reports from WSFS and Business Meeting committees, and it can take direct action on anything that isn't a constitutional amendment. (That includes Hugo eligibility-extension proposals, of which we already have one submitted.)
The PBM can also kill new constitutional amendments by the newly-adopted Postpone Indefinitely process. During the PBM only, the motion to Postpone Indefinitely is in order (even after some discussion or debate has happened on a proposal). It allows four minutes debate (two minutes each side) on the question "Shall the proposal be indefinitely postponed?" and requires a 2/3 vote. A motion that is postponed indefinitely is effectively dead for the duration of the current year's Business Meeting. (It can be resurrected next year; no single meeting can put a subject out of the reach of a subsequent year's meeting.) I'm not going to go into the outlier cases of Reconsidering a motion to Postpone Indefinitely unless someone really wants to know about it.
We still have the motion to Object to Consideration, which can kill a motion without debate the moment it comes up, but it now requires a 3/4 vote, and I will strongly discourage its use except in extreme cases where I think the very consideration of a subject would be harmful to WSFS. (Motions to censure are in this category.) If you want to kill a proposal, move to PI it and be prepared to say in two minutes or less why it's not worth debating the proposal at all.
Day 3 is the first Main Business Meeting. For most of the past twenty years, everything that survived the PBM on Day 2 has been debated and finally voted upon at this meeting; however, this hasn't always been the case, and business has been left over until Day 4 in the past.
Day 4 is the Site Selection Business Meeting, where the announcement of Site Selection is the Special Order of Business, meaning it goes first. Once that's dealt with, we resume consideration of the agenda where we left off the previous day. Because in most years we finished the substantive business on Day 3, this has left nothing but Question Time for Worldcons and bids, after which we adjourn sine die ("without date"), ending the parliamentary session and bringing business to an end. However, there's no reason we have to adjourn sine die on Day 4.
Day 5 is the Final/Overflow Business Meeting. It's in the schedule in case the previous days' meetings are unable to resolve all the business on the agenda. The last time we held a Day 5 meeting was 1992, and that was only because in those days you selected NASFiC by a "snap election" after the selection of a non-North American Worldcon required a NASFiC. (Lead times are different now.) In 1992, we had to hold the meeting open an extra day to handle the selection of a site for the 1995 NASFiC, which is a whole different story about which I might write again some other time.
Just because the Day 5 meeting is rare doesn't mean you can't use it. I have heard multiple people proposing lots of changes to the Hugo Awards, including the idea of passing multiple overlapping and conflicting changes and sending them all on to Kansas City, on the grounds that "We have to pass everything because we won't know how the voting went on this year's Hugos because we adjourn before the Hugos are handed out on Night 4." My response to this is, "Why not postpone the decisions until after the Hugo Awards ceremony?" You can then read all of the stats and see what the results were, and use that to make up your mind about what the appropriate rules changes are.
I would rather have the meeting pass proposals that do not dump a parliamentary snarl into jared_dashoff's lap next year, not the least of reasons being that I'm Jared's deputy in Kansas City. Therefore, anything that allows this year's Business Meeting to produce a non-conflicting set of changes for consideration next year is a good thing in my book.
So it's possible for the meeting to put off consideration of proposals until Day 5, the morning after the Hugo Award Ceremony. How could it do this?
The first way to put proposals off until Day 5 would be to set them up in the Preliminary Business Meeting as "General Orders" with a "Not Before" order on them. That is, you designate that a given item is not to be considered before the Day 5 meeting, or you could put a specific time, i.e. "Not before 10:30 AM on Day 5." This is similar to how tennis tournaments are scheduled. The good part about such things is that people who are only interested in specific subjects know that their hot buttons won't be pushed any earlier than a given time. But that's also considered the bad part by Business Meeting regulars who think you should sit through everything; it also potentially creates a dead spot in the schedule, and I would rule that the motion to suspend the rules and take up a proposal early is out of order because it violates the rights of absentees.
The second possibility is for the Day 3 meeting to debate proposals and collectively determine that the members can't decide without more data that won't be available until Night 4. At that point, they can Postpone Definitely any item into Day 5. This is functionally the same as the agenda-setting at the PBM. Incidentally, note that any debate-time limit set by the PBM resets each day, so if you had 20 minutes set on the debate on a proposal and argued it for 18 minutes on Day 3, then postponed it to Day 5, it would come back up with a fresh 20 minutes.
Note that Postpone Definitely is a completely different motion from Postpone Indefinitely. Both are incidental motions, but PI is the lowest-ranking one and kills the target. Postpone Definitely is a separate motion that outranks PI, Amend, and Refer to Committee (meaning you can move it when any or all of those other motions are pending) and puts off consideration to a specific time.
I hope this explanation makes sense. It gets into a number of the finer points of parliamentary detail, but given the complexity of the tasks we may fact this year, I think it important that people understand what tools they have at their disposal.