In a testcross, the progeny directly show the unknown parental genotype. In this case, where the female parent has a dominant allele, that determines the phenotype in the offspring. Where the female has a recessive allele, that works with the identical allele in the recessive (known) parent) to produce the phenotype.
Work Step by Step
The recessive parent in a testcross is like a white canvas. If the unknown parent as a dominant allele, think of that like a strong color which masks the white and is obvious. A recessive allele in the unknown parent is like white paint which blends in with the canvas--you know it by what you don't see.