It refers to one action from its character John Barr, whose picking up of one of two objects (a magnet or a pebble) is a major turning point in history: choosing one will lead to a utopian civilization named Jonbar, while the other to the tyranny of the state of Gyronchi.
This crucial moment (about which other characters are forewarned and must act), is thus a "Jonbar point" in the novel's timeline, a forking-place upon which hinges the rest of its history.
[3][4] It can, however, refer to any kind of change in history without having to deal with time travel, as Paul Di Filippo used the term when he reviewed S. M. Stirling's In the Courts of the Crimson Kings.
[5] In Michael Chabon's The Yiddish Policemen's Union the jonbar hinge was the death of the main opponent to the passing of the King-Havenner Bill, which would have allowed Jewish refugees to settle in Alaska.
[6] The jonbar hinge in Bring the Jubilee, Ward Moore's 1953 novel of American Civil War alternate history, is the Confederate occupation of Little Round Top minutes before the Union's attempt, which leads to Lee's victory at the 1863 Battle of Gettysburg.