At the stigma - pollen lands pollen fails to germinate. Moisture for germination comes from the stigma. Self pollen is denied moisture. At the style - pollen tube fails to grow when RNA hydrolyzing enzymes enter the tube and prevent protein synthesis. At the ovary.
Fertilization fails or fertilized zygote dies.
The female plant improves her fitness is she discards inbred embryos and puts all of the available nutrients into outbred embryos.