– A force perpendicular to the edge is applied. The force scales with the projection of the relative speed on the edge ( dot product ).
– Note the force is the same if wind is blowing or if you drag the edge through the air with the same speed.
– An edge moving in its own direction feel no force.
– An edge moving perpendicular to its own direction feel maximum force.
– In between you have the forces like they are when you fly a kite.
– you can find a very good explanation here.

– Note: the soft body on the right is falling down faster and faster, the cubes seems to invert their direction but it’s only due to a stroboscopic effect ;-)

– In this second example note how the bigger the Factor values the faster the soft body begins to deform.

Blender Soft Body Aerodynamics Factor