Put the clamps on the horn, and the bars, and put the bolt thru loose enough so you can move the horn. Rotate the handlebar clamp until the long bolt is exactly vertical, or exactly perpendicular to the handlebar. Now the horn in the clamp will turn right or left and will twist perpendicular or 90 degrees to the bar.

In other words you twisted the handlebar clamp until the surface where the clamps touch is parallel to the bars. This allows the horn clamp, and horn, to spin sideways ( move right or left) all the way to perpendicular to the bars.