Why does your butt get red while getting a bare bottom spanking?

During a bare bottom spanking, the repeated impact on the skin causes blood vessels near the surface to dilate and break, resulting in increased blood flow to the area. This increased blood flow leads to the reddening of the skin, known as erythema, as a natural physiological response to the trauma. Additionally, the release of histamines and other inflammatory mediators in the skin during the spanking can further contribute to the redness and inflammation of the area.