Mechanism for synthesis of 4-nitrophenylhydrazine from the diazonium salt of 4-nitroaniline

I am trying to figure out the mechanism to create 4-nitrophenylhydrazine from the diazonium salt of 4-nitroaniline. I've found a couple of explanations for what reactants are used, but I don't understand the mechanism behind them. The mechanisms overall seem to suggest that $$\ce{HCl}$$ is used, and the $$\ce{Cl-}$$ bonds to the $$\ce{N2+}$$. That makes sense, but then it is suggested that $$\ce{Na2SO3}$$ and $$\ce{SnCl2}$$ are used to somehow change this into 4-nitrophenylhydrazine. If anyone could explain how this works, I would be very grateful!

Here is the article showing which reactants are used.

• In the 1st step you also need $\ce{NaNO2}$ and is a standard textbook mechanism. The second step is a reduction. You should correct your reagent to $\ce{Na2SO3}$ – K_P Oct 27 '15 at 0:20