Fin Function - Traction or Thrust

I have been reading threads here in an attempt to understand the function of fins. But I still cannot figure it out.

Do fins create traction so that the surfer doesn’t fall down the wave when surfing parallel to the wave?

OR

Do fins act as a surface against which the energy of the wave is transfered from the water to the board?

Hi, check this … http://www.swaylocks.com/forum/gforum.cgi?post=316573;search_string=the%20function%20of%20fins;guest=11180750#316573