I have been a party to this "phenomenon" twice, which given my generally low-volume purposeful-tickle history is statistically significant.
In college, very briefly "dated" a girl who was, shall we say, filled to overflowing with raging and uncontrolled hormones. Basically, we made out a bunch for a month, usually in one of our respective dorm rooms. There was an incident where she confessed to being turned on when I tickled her feet, but it was said in passing and being a fat, nerdy autistic kid I didn't think anything of it. Fast forward a few days, in her dorm room while her roommate with in her boyfriend's dorm room, we were making out (why is kissing such a thing?), and on a break I pulled off her shoes and socks and started tickling her feet. She was ticklish, but not screaming, thrashing type of ticklish. A few seconds turned into a minute, then two, and her body completely changed. Laughing and wiggling turned into caught breath, shuddering, and full body pulsations. Concerned she was overwhelmed, I stopped and she immediately shoved her feet back into my hands and said "No! Keep tickling!!". I did, and she laid back, clearly getting worked up. She then violently flipped over onto her stomach, put her feet back into my hands, said "Don't stop for anything" and proceeded to thrust a hand down her pants and masterbate while I tickled her bare feet until she orgasmed, stifling her moans and eventual scream into a pillow. Post orgasm she was extremely more ticklish, and pulled her feet away saying she couldn't take anymore. One and done, it wasn't long before we went our separate ways.
Post relationship, late thirties online-dating, met a woman and we had a dinner date, followed by a return to her condo to talk and watch a movie. Eventually her socked feet were beside my leg, I placed my hand on the top of her foot, she flinched. Said her feet were crazy ticklish. I gave her a quick test tickle, and she was extremely ticklish but moaned instead of laughed. I said she'd never survive a foot massage, and she said she'd never even tried. Later I gave her a few test rubs, she laughed but wanted more. Socks came off, bare feet far more ticklish than with socks, but with a lot of care a massage could kind of happen.
The next day, we bantered by text but she asked me to cover over that night. I did, and we were watching "Raiders of the Lost Ark" sitting on her bed. She kept touching me with her bare feet, and finally just said "Will you try to tickle me again?" Okay, I thought. Something must have shifted. So for the entirety of "Raiders", I was in some way tickling her bare feet. She was clearly dealing with a lot of emotions about it, but was not wanting the tickling to stop. After a solid 90 minutes of constant little tickles and trials, she finally just said "keep going....". I did, she pretty much wrapped her legs around me from behind putting her feet crossed in my lap and I tickled. By the time I realized her wiggling around was really her grinding her groin against my back while I tickled her feet she was in too deep to stop. Grinding while having her feet tickled, she quickly reached her climax, gasping into her hands (her daughter and daughter's friend were asleep down the hall), and once again her feet got much, much more sensitive once she orgasmed. But upon realizing it tickled her more, she wanted to keep going. After a couple of minutes it was sensation overload and I stopped.
