The First Time Getting Spanked in Life

Girls who have never been spanked are usually particularly "tender"
The capillaries under the skin haven't been repeatedly stimulated, and the fat layer hasn't been slapped yet to become more able to withstand impact, so the first time being hit by a palm or a paddle, the reaction will be much more obvious than someone experienced.
You can see the difference 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 in the same spot, small bruises easily appear
She can't help but twist her waist, clamp her legs together, or let out short gasps, because the pain and heat surge up at the same time, 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 quickly swell up, the surface becomes hot, feels hot and tight
This "first time" sense of vulnerability is exactly what many people find stimulating
It's not because she's particularly weak, but because her body hasn't been trained yet, every reaction is real, direct, and can't be hidden