What is coupling of reaction? How is it useful in metallurgy?

If the value of $\Delta $G is positive for any reaction, then to make such reaction spontaneous, it is coupled with another reaction of large negative $\Delta $G value, so that the sum of two $\Delta $G becomes negative. This is known as coupling of reaction.
In the metallurgy, thermodynamically infeasible reaction is coupled with a reaction which has more negative $\Delta $G, so that net $\Delta $G becomes negative.