That's an outflow flap.
Women... particularly women more busty than this girl, run into a problem with water getting sucked into the top of the suits when they swim fast, or especially when they dive into the water. This tends to balloon the suit and leave it not fitting properly when the extra water squeezes back out. This lower flap lets the water flow out immediately without creating any wardrobe malfunctions.
Women... particularly women more busty than this girl, run into a problem with water getting sucked into the top of the suits when they swim fast, or especially when they dive into the water. This tends to balloon the suit and leave it not fitting properly when the extra water squeezes back out. This lower flap lets the water flow out immediately without creating any wardrobe malfunctions.