When you assign a trajectory to be a tangent trajectory, a centerline is added to the sweep section's sketch. This centerline is tangent to the neighboring surfaces at the intersection point of the trajectory and the sketch plane.

The tangency is maintained as you sweep the section. Hence, the angle of the centerline changes depending on the location of the section and the neighboring surfaces.

When you select a sketch entity to be tangent to the centerline, you can make the resultant sweep geometry to be tangent to the surfaces adjacent to the trajectory.