First Time Getting Spanked in Life

Girls who have never been spanked are usually particularly "tender"
The capillaries under the skin have not been repeatedly stimulated, and the fat layer has not yet been patted and can thus better withstand impacts, so the first time being hit by a palm or paddle, the reaction will be much more obvious than for those with experience.
The difference can be seen just with the palm:

The first slap down, the skin immediately turns red, after a few hits, the palm print will clearly emerge, color from pink to deep red.
If the force is a bit heavier, or repeated slapping on the same spot, small bruises are likely to appear
She will involuntarily twist her waist, clamp her legs together, or emit short gasps, because the pain and heat surge up at the same time, and the body hasn't learned how to process it yet

After switching to a paddle, the difference is even more obvious:
A butt that hasn't been spanked will swell up quickly, the surface becomes hot, feels hot and tight to the touch
This "first time" sense of vulnerability is exactly what many people find stimulating
It's not because she's particularly weak, but because the body hasn't been trained yet, every reaction is real, direct, and can't be hidden