Because you move the cable back as you twist the fwa out. So the travel of the clutch cable is less.
Going to make up numbers here
Lets say the factory cable set up gives you 5'' of cable under the dash. It takes pushing the clutch pedal an inch from the floor before the clutch is fulling disengaged.
Now add a FWA and quadrant to it. Before adjusting the fwa out you still have 5'' of cable under the dash. But this time you can twist the FWA outward. You twist the fwa outward moving 1'' of cable from under the dash and now it's between the firewall and the opening of the fwa. With an inch of slack now gone, the clutch peddle now only has to move halfway before the clutch is disengaged.
Making any sense?