As this allows for a much more rapid vaginal birth, there is an increased risk of tearing of the perineal soft tissues. At the time of delivery, the strong uterine contractions and maternal bearing down almost instantly overwhelm the integrity of a previously unstretched vaginal orifice.
A faster vaginal birth is possible due to this method, however, there's an increased chance of tearing the perineum during childbirth. The powerful uterine contractions and the mother's efforts at pushing almost immediately overwhelm the flexibility of the vagina, which was not stretched beforehand.