That has a simple explanation. The lift rod (248) is made of two interconnecting parts with a compression spring in the middle. It is the compression of these two parts of the lift rod via the setting of the adjuster barrel (68) which allow the arm to move. Here's why:
The main lever (173) pushes the lift rod up to enable horizontal travel (arm is clear of record and rest post). If this pushing force is not controlled/adjusted/reduced by the barrel(68), then the lift rod, with the upper pimple (66) on top, will push the arm onto the adjuster bolt (60). The arm can't move horizontally because it is held tight by bolt 60 and pimple 66. Now the lower pimple (239) simply can not overcome this hold and simply slides across the surface of the main lever 173 instead.
By decreasing the amount of lift with the barrel, the arm will not be locked horizontally, thus allowing the lower pimple to hold and not slide on the main lever and thereby move the arm horizontally. The adjusted amount of freedom to be set by the barrel is shown in Fig 25 as previously stated.
Hope this explains it a bit better,