When the two species are mixed, the driver sequence is added in excess to tester.
During PCR, double stranded fragments first denature at ~95 °C and then re-anneal when subjected to the annealing temperature.
Since driver and tester sequences are nearly identical, the excess of driver DNA fragments will anneal to homologous DNA fragments from the tester species.
This blocks PCR amplification and there is no increase in homologous fragments.
However, fragments that are different between the two species will not anneal to a complementary counterpart and will be amplified by PCR.