My take on fins has only two aspects. Lift and drag. The lift isnt against gravity its against leeward movement.(But drag has two aspects.the drag induced by causing lift and parasitic (frictional) drag.???)
We picture a fin going straight through the water but it doesn`t. It goes very slightly sideways.(Its why they need very rounded and not pointed leading edges)
That sideways movement (caused by leeway itself caused by the wind) transforms the "skeg" into a foil ,generating a force against leeway.(Imagine the foil going through the air with the leading edge higher than the trailing; it becomes a wing and generates lift.If it was level (ie leading edge same height as trailing edge it would not produce lift.(dont forget our fins are symetrical unlike a plane wing)
If you tow a board in a straight line the fin produces no lift; it only produces lift when made to "crab" through the water.Towing in a straight line only has drag.(and only parasitic drag)
WS is great..