Directional shear is a change in wind direction with height, and it is the primary source of hodograph curvature. When the winds turn clockwise with height, the profile is veered. When they turn counterclockwise with height, it is backed.
How it shapes a storm
The horizontal vorticity generated by directional shear is oriented differently at each level. In a strongly veered profile, that vorticity aligns with the incoming inflow, which is what forecasters mean by streamwise vorticity. An updraft ingesting streamwise vorticity stretches it into the vertical and becomes a rotating updraft, or mesocyclone.
By contrast, purely straight-line shear produces vorticity oriented across the inflow, which is called crosswise vorticity. That configuration favors a splitting pair with counter-rotating updrafts, not a single dominant rotating cell. The difference between an environment that produces splitting multicells and one that produces cyclonic tornadic supercells is largely how much the wind turns in the lowest kilometer.
How forecasters use it
The most operationally useful reading of directional shear happens on a hodograph rather than by comparing two wind barbs. A large, curved arc in the 0 to 1 km layer, especially one that turns from southeast to south to southwest with height, is the classic signature of a tornado environment.
In synoptic terms, southerly veering to southwesterly winds aloft are supported by warm advection near a surface warm front, and by an approaching upper trough that brings west-southwesterly flow overhead. Chasers often target the intersection of a warm front with a dryline or triple point because that is where enhanced backing near the surface, and the warm-front-induced turning above it, stack together.
Important limits
Directional shear only helps if the storm is drawing from the layer where the turning is happening. If a shallow cool boundary layer isolates storms from the sheared inflow, the hodograph above may look impressive while the storm sees a much simpler profile. Effective-layer parameters were developed in part to catch this mismatch.
Backing at just one level is also not the same as a favorable profile. Elevated backing near the top of the boundary layer can produce alarming shear on paper without translating to any storm-scale rotation, because the storm's updraft is not tapping that layer. The full hodograph shape, not any single level's turning, is what matters.