Understanding NLSolvers Package through the Given Documentation

I am quite new to Julia and trying to figure out how to use the NLSolvers package as given in the following link: NLSolvers

In the README file, they provide an example for a nonlinear system of equations for the Rosenbrock model problem. I understand what the first two functions are doing – providing the equations and then the Jacobian of the system.

My confusion is about the last function: Jvop_rosenbrock!. I cannot figure out what this is and how they determined what Fv should be.

I want to take this example and apply it to a nonlinear system of 6 equations. Any ideas on what this function is doing and how it relates to the Rosenbrock problem. Thanks!