I've considered learning this run, but i'm a little confused what State Swapping is. Does State Swapping mean you can not Load another Save state (witch is obv. i think) ? Or is it some kind of tech?
An answer would be Welcome. Thanks in advance

It is a glitch where you can use a save (made during the run itself) to advance the current quest to the next one, allowing you to skip fights and quests. It doesn't work for every single fight and quest, but you can skip a vast majority of the game with doing this trick over and over again.

could you make a vid of it

not my vid, someone else made video of it and found it.

Thank you Oh_Snap!

It seems it saves a lot of time but its really annoying. Why should somebody run this?

Maybe some people like to just do stuff what breaks the game, i personally dont see the appeal.