Where did this come from, I searched it in discord gifs and everything looks so odd! 😭
I have started to see on twitter a lot of artworks/cosplays of girls/boys anime/irl doing this pose of their butt in the air and the hands on the ground and camera pov from the front.
Why is it called jacko? Where it's started by who and if possible why?
https://mobile.twitter.com/search?q=%23thejackochallenge
I have would have posted a link from reddit (bc i also saw a lot of those in there) but i couldn't find not nsfw.
Answer: Jack-O is a character from Guilty Gear Xrd Rev 2. (Yes, that's what they called it. Guilty Gear number conventions are weird.) https://dustloop.com/wiki/images/0/0a/GGXRD-R_JackO_Portrait.png
She has that stance as her crouching animation and it's kind of been a meme for how...well, how back breakingly absurd it is.
The original pose: https://i.kym-cdn.com/entries/icons/original/000/038/065/Jack-O_Crouch.png
She was recently announced as the newest character for Guilty Gear Strive, https://www.youtube.com/watch?v=qVOe7IRrlhY and she still retains that same crouch animation. So people have latched onto it.
Answer: from clicking your link I found out real quick, the challenge is named after a character from the fighting game Guilty Gear who in one of their poses does the exact pose, people saw how difficult the pose looked due to the flexibility and made it into a challenge, then the art came in because it was a trend and artists want to do well on Twitter.