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The arrow in the Unbend function sets the material side, so determines whether the skin you are working on was "inside" or "outside" of material.  Which is important as the outside when unbending is elongated material, and the inside is compressed material.  Arrow side has a pronounced affect on the end length of the unbend, but it works also in conjunction with the K factor value, which is where inside the material thickness the neutral plane is.  So flipping the arrow will move the neutral plane either over or under the faces you are working on.

Explanation of K factor:

Now consider a case like this, 6 identical simple bends:


Given the settings, The bend radius varies in all 6 cases, as the neutral plane is moved farther away from the surfaces, and also if the material side is flipped by flipping the arrow.  The overall length then must also be different.  The blue line is drawn at inside material, .5 K factor as a length reference.