How do male dunnocks reduce the amount of rival sperm in females before mating?

In the context of reproductive behaviour and sperm competition, male dunnocks (Prunella modularis) employ a specific physical action to improve their chances of paternity when mating with a female who has recently copulated with a rival.12

Before a new male begins to mate with a female, he pecks at her exposed cloaca. This stimulation causes the female to eject a droplet of semen that contains the sperm of previous mates. By removing this rival sperm, the male increases the probability that his own sperm will successfully fertilize the female's eggs.132