Even as a young child, I always fantasized about being saved by various female characters from video games, whether or not they themselves were going to be hurt or not. In fact, I'm pretty sure I had more of a gyaku fetish back then, mainly because I was exposed to male characters primarily (I have Mario 64 and Pac-Man World yeah, really, let's leave it at that to thank for that).
It developed more towards women once I started playing more games with females in them. I remember getting the opportunity to play an arcade fighting game, picking a female character, and just watching her get beat up and knocked out. I didn't quite understand why, but I liked it. I also remember going over to a friends' house to play Melee (my first time, actually) and playing a Stamina Match as Zelda. I can still picture her body falling flat onto the stage as her death cry echoed. Stamina Mode holds a special place in my heart.
However, tt really started with Crash Bash (strangely enough). There was a brawler-type minigame in which, when a character ran out of health, they would fall onto the ground, unconscious, before transforming into the angel figure the series was known for.
Anyway, I would pick Coco and just watch her get beaten up, listening to her grunts and finally seeing her fall onto her back and her head facing away from the camera (sometimes, I'd see the front of her face, there was no closed eyes texture for this).

It really kicked off as I discovered RPGs (like Square and Nintendo's) that featured characters that would actually fall unconscious (a lost art in Square's RPGs today). Emulation gave me the opportunity to try all kinds of games and see which ones would actually depict females unconscious. Seeing Terra in screenshots of Final Fantasy VI convinced me to try the game and see what her KO pose looked like.

I later discovered "ryona" through Tomb Raider death videos, stumbling across FallenParty's videos, then finally seeing a video with ryona in the title. The a-ha moment felt incredible, yet strange.
So, in a nutshell, I'm most thankful for the existence of both the KO and sleepy ryona fetishes.