Why does Mg react vigorously in NaCl solution and less so in water?

When you put $\ce{Mg}$ into water a few $\ce{H2}$ bubbles appear. But when you put $\ce{Mg}$ into a $\ce{NaCl}$ solution there is a vigorous release of $\ce{H2}$, why is this and what reactions are taking place?




It would still be the same reaction.


\begin{align}
\ce{Mg (s) &-> Mg^2+ + 2 e-} & E^\circ&=\pu{2.36 V}\\
\ce{2 e- + 2H2O (l) &-> H2 (g) + 2OH-} & E^\circ&=\pu{-0.828 V} \times 2
\end{align}


The combined reaction would be spontaneous ($E > 0$), but $\ce{Mg(OH)2 (s)}$ is highly insoluble, 
\begin{align}
\ce{Mg(OH)2 (s) &<=> Mg^2+ (aq) + 2OH- (aq)}, & 
K\_\mathrm{sp}=7.1 \times 10^{-12},
\end{align}
leaving the solution insufficient ions to conduct the electrons necessary to fast complete the reaction (despite of the escaping hydrogen gas). 
$$\ce{Mg (s) + 2H2O (l) -> H2 (g) + Mg(OH)2 (s)}$$


This is like an electrochemical cell with no salt bridge (or bridge with only de-ionized water). Hence a little salt ions would accelerate the reaction forward.


In contrast, calcium solid, having comparable standard reduction potential, will undergo a similar redox reaction in water and needs no help for additional ions because $\ce{Ca(OH)2 (s)}$ is about a million times more soluble ($K\_\mathrm{sp}=6.5 \times 10^{-6}$) than magnesium hydroxide, providing sufficient free ions in the solution to drive the vigorous release of hydrogen gas. (There are a couple of YouTube video demos.)


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