finasteride works by binding to the 5-alpha-reductase (5AR). 5AR interacts with testosterone to produce DHT, which binds to the roots of the hair follices, thereby damaging the hair. Incidentally, DHT also binds to sites in the prostate, which is why 5AR is also useful for prostate cancer.
With most steroids, 5AR will not even interact with the molecule, therefore no transformation takes place.
With Deca, however, 5AR transforms the nandrolone molecule and transforms it DHN, the nandrolone analog of DHT.
It should be pointed out that both testosterone and DHT bind to the hair follicles, but DHT binds, I think, 5x more "tightly" (anyone know if this is 5x the binding energy? 5x the average binding lifetime? etc.). So, prevention of the test->DHT transformation is desirable for hair. In the case of deca, DHN binds less tightly than nandrolone. Therefore prevention of the deca->DHN transformation is *not* desirable.