I've heard that at the beginning, it's better to have the elbow at a fixed position so that the shots become more consistent, with using the leg to adjust to the ball rather than the arm.
https://youtu.be/hybQkZMUifo
At the start of the video, you can see that Ma Lin keeps his elbow fairly close to his body when looping.
https://youtu.be/A5lmr4FoqH8
In this video, Xu Xin's forehand looks almost fully outstretched.
Which one do you think is better for beginner players?
Xu Xin doesn't stretches his arm either. He just has a more fixed elbow position. From another angle they would seem more similar. fixed elbow and wrist is good. It gives you more consistency. But don't do it hard! The key points:
you shouldn't have tension in you arm or shoulder or wrist when you start the swing, the loop should feel natural. If you feel you're slow, then the problem is with your weight transfer.
loop forward!! look at fang bo in the video you posted
weight transfer.
accelerate at the contact!!!!! it will increase the quality of your loop in every aspect: spin, speed, control. tons of people I see to play are trying to swing hard. I did it too. That's not the right way. the swing should feel loose and natural. of course you mustn't just swing it totally free, a loosely fixed elbow and wrist is needed, but avoid having too much tension. When you're about to contact the ball: BANG! It's a short, intense action. it's quite hard to notice in the videos, but it's there. When you hear Fang Bo breathe out when they are counterlooping in the video you posted, that's the moment when he adds power) You will feel when you doing it right.
EDIT: Over time, when you're more consistent you want to add more wrist action. some people is talented in this and able to use wrist correctly from early on, some just messes up his loop with his wrist, either with not using it or overusing it. I would recommend you to don't pay attention to your wrist other than not having it totally locked and tense.
EDIT: +1 point: don't contact the ball at it's back! it will either cause a spinless ball or encourages a bad upward looping motion
3rd EDIT, I promise this will be the last
: you might see that the chinese players let their arms down before the loop, especially FZD.I would recommend you to avoid letting your racket below waist level, because it also encourages upward motion. It's not that it's wrong, just FZD and other chinese know the right technique. however, unless you're very conscious during training and train a lot it might build bad habits when you aren't aware of it. here is Harimoto looping, he is a good player to imitate:
https://www.youtube.com/watch?v=1sGQz5Xp7uE