It indicates that one's nerves become braced by having a rokushaku fundoshi clenched between the buttocks and fixing the genitals, and that the fastening of the garment makes it easy to put strength in the lower abdomen.
つまり、尻に食い込み陰部を固定させる事により気持ちが引き締まり、締め付けることで下腹部に力を入れやすくする。