leverage.plot() R - CAR package

I'm trying to complete a homework regarding added-variables and leverage plots using the CAR package.

In the documentation of the leverage.plot() it says that "These functions display a generalization, due to Sall (1990), of added-variable plots to multiple-df terms in a linear model. When a term has just 1 df, the leverage plot is a rescaled version of the usual added-variable (partial-regression) plot."

I really don't understand what that means. Does anyone know what's the difference in the interpretation of a leverage compared to a partial-regression plot?