The kitten will stop moving after its neck is grabbed. There are several main reasons for this.
First, this is a conditioned reflex. Kittens are mammals. Newborn kittens have no behavioral capabilities. Their eyes are closed. They rely on their mother's milk to survive. Nature is constantly changing. Kittens often encounter many dangers. They also frequently migrate. Many parts of a kitten's body are very vulnerable. The flesh on a kitten's neck is very loose. If the mother cat wants to carry a kitten, she will grab it by the flesh on its neck and carry it away. 
Kittens shouldn't move around too much at this age. If they do, their mother cat's teeth will scratch them. The kittens gradually adapt to this behavior. When they grow up, they won't move if someone grabs them by the neck. This is actually a conditioned reflex. They must be remembering their mother at that moment.
Secondly, the kitten's body tenses up, and it becomes unable to move. A kitten's flesh is very loose, and its bones are soft. It is also very agile. A kitten's weapons are its teeth and claws. If we grab a kitten head-on, it will attack. However, if we choke it, it will become completely still. When we choke a kitten, its entire body is tense. The kitten's forelimbs will remain stiff.
If you move it at this time, it will feel very uncomfortable. If you are the cat's owner, it will also relax its hostility towards you. The kitten will also accept you choking it. If you are not the kitten's owner, you should not do this dangerous action. It will frighten the kitten. If your cat is already quite old, you should also not do this action. The cat will feel pain.
