Fixing directions allows you to adjust the orientation of the contour during its sweeping along the trajectory.
Faces, 3D paths, 3D profiles, etc. can act as a fixing direction. You can also specify a pair of points that form a direction vector.
Vector fixing the direction should not coincide with the tangent to the trajectory at any point of the trajectory.
Fixing by path and direction: tangent to the trajectory at each point of the trajectory is projected onto the plane perpendicular to the fixing direction. The coordinate system of the intermediate section is constructed from obtained projection of the tangent to the trajectory and fixing direction.
Fixing of rotation: the coordinate system of the intermediate section is constructed by the tangent to the trajectory at the given point of the trajectory and by the cross product of the fixing direction vector and the tangent.
Source operation without using fixing direction 

Fix by Path and Direction 
Fix Twist 