As a girl I was scared/thrilled at seeing any of my siblings/step siblings get spanked but hated my own spankings, especially the embarrassment of being watched by witnesses.
At 12-yrs-old I saw my 15-yr-old stepsister take a serious bare bottom hairbrushing from her mom and I got super excited at seeing a "big" girl bawling and kicking like mad. I remember being amazed when she did a spanking dance and I could see her full pubic bush and I could see how humiliated she was at me being there to see her spanking.
After that, I took every opportunity to watch any spanking I could as the cute bare boy or girl bottoms turning red and their screeching and kicking was thrilling! At that point I still HATED any of my own spankings.
But at 16 a girlfriend and I got caught smoking at her house. My mother gave my friend's father permission to spank me along with his daughter. We both had to strip to bra and panties , right in front of my friend's mother and 13-yr-old brother! I was mortified but also really wanted to see my friend get spanked as she was super cute and always acted "superior" to me, plus the cigs were HER idea!
Her big, strict Dad took her panties down and lectured her, then took her over his lap for a wicked bare bottom belt whipping. Watching her twist and buck and kick as her perfect tight bubble butt got whipped and hearing her begging, screaming, bawling was so exciting!
And seeing her mom and her brother watching with slight smiles should have made me crazy with shame, but somehow knowing they would see me half naked and howling added to my building excitement! When she was sobbing in the corner and I got called to him, my nipples were stiff as nails thru my bra and my panties were damp.
When this handsome man I hardly knew was pulling my panties down, his face just inches from my bush, I almost swooned and his eyes looked me up and down while scolding me and I glanced around to see the mom and the boy staring at my body.
The whipping was bad and I squirmed and jerked and I remember kicking my panties off and spreading my legs wide open.
My "thrill" disappeared under the wicked whipping but once I was up and dancing I ended up jumping around rubbing my burning buns longer than necessary because I liked the feeling of being the center of attention, especially with a man and boy so obviously delighted to see my front fur and bottom and my boobs bouncing in my bra.
So, for me, the biggest trigger is the fear/shame of bare bottomed spankings with witnesses, either with me watching or being watched.