Also known as: 水抜き穴
The frontal flap of a School Swimsuit (intended as a water outlet while swimming), either being pulled open or otherwise displayed as a focus of the image.
These flaps only appear on Old School Swimsuits; if it doesn't have a flap, it's a New School Swimsuit.
This tag does not apply if the swimsuit bottom is merely pulled to the side to reveal the genital area; use Swimsuit Aside for that instead.