The perturbation plot helps to compare the effects of all the factors at a particular point in the design space. The response is plotted by changing only one factor over its range while holding all the other factors constant. By default, Design-Expert sets the reference point at the midpoint (coded 0) of all the factors. You can change this to be any point (perhaps the optimal run conditions) by using the Factors Tool.
For a more general set of graph tools see Model Graphs - General Information
A steep slope or curvature in a factor shows that the response is sensitive to that factor. A relatively flat line shows insensitivity to change in that particular factor. If there are more than two factors, the perturbation plot could be used to find those factors that most affect the response. These influential factors are good choices for the axes on the contour plots.
Remember that this plot is like “one factor at a time” experimentation – it does not show you the effects of interactions!