The double rod is installed directly under the fretboard in a straight trench approximately 3/8" deep. A long adjusting nut is used to focus the truss action away from the unmovable heel joint.
When flexed in the neck, resulting pressures are shown above. The blue arrows show the two rod ends exerting high downward forces while the middle pushes up uniformly. The resulting neck movement shown with the yellow arrow, determines string action height and corresponding relief.
The force exerted by the tail end of the double rod is considerable. There must be a minimum 1/8" of wood below the rod end to prevent wood failure in this area. This becomes a concern only when thinning a neck beyond factory engineered dimensions.
Benefits of the double expanding truss rod