When you move through water you create a 「bow wave」 with your head and body, just like a boat does. The shape of the bow wave means the water level drops along the side of the swimmer's face.
当你随着你的头部和身体在水中移动的时候,你会随着产生一股「涡流」,类似小船产生的一样。涡流的形状表示水位沿着游泳者的面部两侧下降。