The BM consists of three meetings, usually held from 10 a.m. to noon on the three middle days of the convention (Friday, Saturday and Sunday for a "normal" Worldcon which ends on Monday).
On the first day, the Preliminary BM hears WSFS committee reports and prepares the agenda for the second meeting. Nominations to the open seats on the Mark Protection Committee) are sought. (In addition, each Worldcon appoints a member to the Mark Protection Committee.) And volunteers are allowed for the Nitpicking and Flyspecking Committee, the Worldcon Runners Guide Editorial Committee, and the the Hugo Eligibility Rest of the World Committee (HEROW).
Preparing the agenda includes taking business which has been submitted, possibly rejecting some of it as not worth considering, possibly merging motions, considering and perhaps passing changes to the WSFS Standing Rules, making other procedural motions, and setting time limits for debate.
On the second day, the formal business session, submitted motions are debated, amended and passed or defeated. Since debate time is strictly limited, it's difficult to hold a substantive debate at this meeting, so usually the issues involved have already been widely smoffed in fandom — in fanzines, on email lists like the Smofs list, on social media and blogs and at smaller cons.
On the third day, the site selection meeting. The site selection administrator presents the results of the vote, the BM ratifies it and orders the ballots destroyed, the winning bid makes a presentation, and the meeting then moves to Question Time wherein the upcoming Worldcon and Worldcon bidders make brief presentations and take questions form the people present. In theory, this meeting could consider business, but this practically never happens. (In fact, there is provision for a fourth session of the BM, just in case; the first time it was needed was at Sasquan in 2015, to consider proposals to prevent Puppification.)
Participation in the WSFS BM is limited to smofs attending members on site at the current Worldcon, though any member may submit business. Attendance varies, but is usually between 100 and 200 people, most of whom attend every year. Proxies are not allowed.
The Business Meeting is governed by Roberts' Rules of Order as modified by a few special practices adopted by WSFS. Proceedings are fairly formal with strict time limits on debate.
Originally, when Worldcon had a single track of programming, the BM was a one-hour item in the program presided over by the chairman of the Worldcon. It voted on the rules and selected the location of the next Worldcon in an informal process with its rules varying considerably from year to year. During the 1960s, the WSFS Constitution became formalized and by the beginning of the '70s, the WSFS Business Meeting had developed into a formal process for making the governing rules.
Worldcon site selection moved to a combination of a mail and an at-con ballot under the management of a site selection administrator, and the role of the WSFS BM changed to one of formally accepting the results of that vote rather than of conducting the vote in the meeting.
Additionally, with the development of multi-track programming, the WSFS BM ceased to be a plenary meeting of the entire convention membership and became a meeting of just those members who show up. Many fans never attend the BM, some attend only when a proposal of special interest is on the agenda, while others would never miss it.