When subject to torsion this profile twists around a center axis defined by intersecting Y and X planes. As shown in Fig 2 the position and orientation of all the external surfaces of the rim shift considerably, while the neutral plane remains nearly unchanged (neutral).
Shown in fig 3, when a rim is formed (hooped) the x-axis (green) changes little as it remains substantially a plane bisecting the rim into left and right (drive side and non-drive side). The yellow plane (y-axis) transforms into a cylindrical form we call “section neutral plane.”
As we form (hoop) the rim and the Y plane transforms into a cylindrical form, things become interesting. Now we can consider how loads applied from various directions deflect the rim. The geometry resisting this deflection is sectional properties and the radius of this section neutral plane. Local loads (individual spokes, drive side for instance) can easily place a significant torsional load as well as the expected radial load. With few exceptions, spoke loads create a mix of torsional axial and radial forces.
This is where it starts to get interesting, as we form the rim we transformed the Y plane into a cylindrical form. Now we consider various loads applied from various directions will deflect the rim, the geometry resisting this deflection is sectional properties and the radius of this section neutral plane. Local loads (individual spokes, drive side for instance) can easily place a significant torsional load as well as what would normally be the effect of an axial load theoretically hoped for. with the exception of paired spokes recently reintroduced by Rolf, the spoke loads will always transition to a mix of torsional axial and radial.
Aligning spokes to the intersection of these two planes is the most direct method of moving the rim with a spoke while truing a wheel. In Fig 5, a spoke hole is drilled with an 8° axial offset, aiming the spoke axis to the intersection of these two planes. This reduces torsional inputs seen while truing a wheel, assisting in repositioning the rim as near axial as possible considering you are putting an offset load on the profile.