good question. cats are induced ovulators meaning they need stimulation in order to release an egg. When the male bites the back of the neck of the female to keep her still during copulation, this will induce but not always. Therefore the withdrawing of the penis may also induce ovulation. The first time doesn't always do it either so a female may have many males breed with her and the outcome is lots of kitties with different daddy's.
Wow!! Learn something new everyday.