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.](http://nopr.niscair.res.in/bitstream/123456789/7404/1/IJEB%2048(3)%20265-268.pdf "The article showing what reactants are used.")


[![Diazonium salt of 4-nitroaniline](https://i.stack.imgur.com/qRjgd.png)](https://i.stack.imgur.com/qRjgd.png)


