It basically works based on player animations (at least to my knowledge).
You can infinitely kata someone over your shoulder by doing the kata just as the player is getting up from a knock down, this works best on classes than cannot roll, but it's not impossible to do on those that do. I have done it many times.
My understanding is that by stalling an animation yourself, or kataing someone mid animation skips the animation on your behalf, giving you the "instant kata".
Doing it over the shoulder is the easiest way, however you can also run up to someone and instantly kata them if you time it right. You can instantly kata someone basically if you kata them while they're mid animation, so anything like a punch, or a swing. You might have seen this while you have played randomly where someone will get a lucky instant kata.
You can stall your own animation, but it's unreliable, since it can be easily disturbed. You can achieve this by using a combination of the "use" "reload" and "block" keys. Though it's pretty inconsistent.
It used to be easy many years ago. All you had to do to stall your animation was hold the "use" key, the community had a term called "hugging" where by jumping and holding "use" your player would look like he was hugging, and you could just run up to people and instantly kata them.
In the video you can see i wait for the audio sound of the Mandalorian getting up and then doing my kata. This is because he is mid animation, i believe the animations just clash and quickly inputs the next animation, which in this case would be the kata.