This article explains it.
http://lists.jammed.com/ISN/2003/06/0104.html
The combination of sgid bit set and execution bit cleared would not make sense normally. That's why it was chosen.
The kernel source Documentation has the locks.txt and manditory.txt files. The book "Advanced Unix Programming" ISBN 0-13-141154-3 has example code in section 7.11.5.
When you open the file, you also use the S_ISGID permission flag:
ex:
perms |= S_ISGID;
fd = open("file", O_CREATE | O_RDWR, perms )
Then use the fcntl or flock command as you would to set an advisory lock.
This is an excerpt from the book I mentioned.