Has a 10% chance of causing the target Pokémon to flinch when using a move that deals damage and does not already have a chance to flinch ~ Bulbapedia
The flavour text does not specify a physical or special bias, so it's safe to assume that Razor Fang is able to cause the foes to flinch when using a Special move. Note that it's not only physical moves that flinch, certain special moves like Extrasensory can as well.
