when you attach the top of the bar to the upper kingpin bolt. But take care because that fine thread can be difficult to restart if there's the slightest shift in the kingpin. .
To stop the kingpin from moving, stick the jack under the bottom kingpin plate. Just jack it enough that the springs cant push the top and bottom crosshead tubes apart (this is why the kingpin can pop out of its recess in the top lug forging), not enough to take any significant weight off of the axle stands
