The male cat's ‘penis' is covered in hook-like barbs, and as he withdraws these barbs abrade the females cat's ‘vulva'. She will scream and turn to bite him. An experienced male cat will maintain control of her until he fees it is safe to release her and move away.
It may sound horrible but these hook-like barbs are essential in causing the female cat's ‘vulva' to be stimulated so as to release the eggs. One mating often is not enough to cause the ‘vulva' to release the eggs; therefore, in some circumstances 2 or more matings are needed.